Reactions to the September 11 attacks included condemnation from world leaders, other political and religious representatives and the international media, as well as numerous memorials and services all over the world. The attacks were widely condemned by the governments of the world, including those traditionally considered hostile to the United States, such as Cuba, Iran, Libya, and North Korea. However, in a few cases celebrations of the attacks were also reported by Canada, and some groups and individuals accused the United States in effect of bringing the attacks on itself.
Many countries introduced anti-terrorism legislation and froze the bank accounts of businesses and individuals they suspected of having connections with al-Qaeda and its leader Osama bin Laden, the accused perpetrators of the attacks.
In the immediate aftermath of the attacks, the Bush administration declared a war on terrorism, with the stated goals of bringing Osama bin Laden and al-Qaeda to justice and preventing the emergence of other terrorist networks. These goals would be accomplished by means including economic and military sanctions against states perceived as harboring terrorists and increasing global surveillance and intelligence sharing. Within hours after the September 11 attacks, Defense Secretary Rumsfeld speculated on possible involvement by Saddam Hussein and ordered his aides to make plans for striking Iraq; although unfounded, the association contributed to public acceptance for the 2003 invasion of Iraq. The second-biggest operation of the US Global War on Terrorism outside of the United States, and the largest directly connected to terrorism, was the overthrow of the Taliban rule from Afghanistan, by a US-led coalition.
American Muslims utterly condemn the vicious and cowardly acts of terrorism against innocent civilians. We join with all Americans in calling for the swift apprehension and punishment of the perpetrators. No political cause could ever be assisted by such immoral acts.
NATO: NATO held an emergency meeting of the alliance's ambassadors in Brussels. The secretary general, Lord Robertson, promised the United States that it could rely on its allies in North America and Europe for assistance and support, and pledged that those responsible would not get away with it.
European Union: European foreign ministers scheduled a rare emergency meeting the next day of the attacks to discuss a joint response, as officials expressed solidarity with the United States. The external relations commissioner, Chris Patten, called the attacks "the work of a madman."
Greenland: People gathered in Nuuk, and other town squares to light candles and offer prayers.
Sweden: Trams and busses in Sweden came to a halt out of respect for the victims.
Norway: Trams and busses also halted in Norway out of respect.
Finland: Busses and other public transportation in Finland came to a stop to pay tribute to the victims of the attacks.
France: The French newspaper of record, Le Monde, ran a front-page headline reading "Nous sommes tous Américains", or "We are all Americans". Following the attacks, French president Jacques Chirac released a statement: "It is with great emotion that France has learned of these monstrous attacks—there is no other word—that have recently hit the United States of America. And in these appalling circumstances, the whole French people—I want to say here—is beside the American people. France expresses its friendship and solidarity in this tragedy. Of course, I assure President George Bush of my total support. France, you know, has always condemned and unreservedly condemns terrorism, and considers that we must fight against terrorism by all means."
Germany: In Germany, Chancellor Gerhard Schröder described the attacks as "a declaration of war against the civilized world." Authorities urged Frankfurt, the country's financial capital, to close all its major skyscrapers. The new Jewish museum in Berlin canceled its public opening. In Berlin, 200,000 Germans marched to show their solidarity with America.
Poland: In Poland, firefighters and other professional rescue workers sounded their vehicle sirens, letting loose a collective wail one warm afternoon. Many Poles also expressed their sympathy by lighting hundreds of candles in front of the U.S. embassy in Warsaw.
Italy: In Italy, race car drivers preparing for the Italian Grand Prix silenced their engines out of respect for the victims of the attacks.
Greece: Greek Prime Minister Costas Simitis expressed his dismay of the attacks on the United States. quoting "Greece condems, most categorically, these horrific acts. We hope that the culprits be located and brought to justice immediately." Many Greek citizens called the U.S. embassy to offer their support and express their outrage over the attacks. Security was also amped up at American and other European embassies in Athens. Opposition cadidate Kostas Karamanlis was in the United States at the time, attending the opening of a Greek Studies Department at Tufts University in Boston. Karamanlis also condemned the attacks.
Bulgaria: People in Bulgaria also gathered in town squares to light candles and pray.
In 2003, New Zealand began administering a "Pacific Security Fund" to vulnerable nations in the Pacific region aiming at securing and preventing terrorism from entering the region, there is an annual fund of NZD$3 million that is paid by the New Zealand Ministry of Foreign Affairs and Trade (MFAT) and is used to provide support to Pacific Island countries.
Mexico: Mexico increased its security, causing enormous traffic jams at the United States border and officials said they were considering closing the entire border. President Vicente Fox expressed "solidarity and our most profound condolences".
Ukraine: As a nation, Ukraine immediately declared solidarity with the United States, and offered moral, technical and military support to the extent of their infrastructure. The Ukrainian parliament passed three resolutions all in favor of assisting the United States following the attacks. Congressman Bob Schaffer expressed gratitude towards Ukraine and its stance on terrorism, saying "Ukraine's condemnation of international terrorism, its much-appreciated support in the war on terrorism, its tough newly enacted laws to combat terrorism, and its commitment to fight at the side of the United States and its allies for civil society and democracy demonstrates the role Ukraine and her people intend to play in the emerging democracy." 
A museum panel showing headlines on September 12 in America and around the world. Most of the images on the headlines are images of United Airlines Flight 175 hitting the South Tower.
United Nations: In the following day of the attacks, the United Nations Security Council members condemned the terrorist attacks on the United States and adopted resolution 1368 (2001), by which they expressed readiness to take all necessary steps to respond to the attacks of September 11 and to combat all forms of terrorism in accordance with their Charter responsibilities.
Then-Secretary General of the UN Kofi Annan, said: "We are all traumatized by this terrible tragedy."
Renowned Muslim scholar Yusuf al-Qaradawi denounced the attacks and the unprovoked killings of thousands of American civilians as a "heinous crime" and urged Muslims to donate blood to the victims. He did, however, criticize the United States' "biased policy towards Israel" and also called on Muslims to "concentrate on facing the occupying enemy directly", inside the Palestinian territories. The alleged Hezbollah "spiritual mentor" and Lebanese Shia cleric Mohammed Hussein Fadlallah condemned the attacks.
Ahmed Yassin, the spiritual leader of Hamas, harshly condemned and denounced the attacks and also stated that he was not interested in exporting such attacks to the United States, however he criticized the "unfair American position".
Afghanistan's Taliban rulers condemned the attacks, but vehemently rejected suggestions that Osama bin Laden, who had been given asylum in Afghanistan, could be behind them.
Huge crowds attended candlelit vigils in Iran, and 60,000 spectators observed a minute's silence at Tehran football stadium.
Polls taken by Saudi owned Al Arabiya and Gallup suggest some support for the September 11 attacks within the Islamic world, with 38% believing the attacks to be not justified, while 36% believing them to be justified. A 2008 study, produced by Gallup, found different results with 7% of the sample of Muslims polled believing the 9/11 attacks were at least 'completely justified', while 23.1% believed them to be 'somewhat justified'.
A group of Palestinians were filmed celebrating in the street in celebration of the attacks on the World Trade Center and the deaths of Americans. Fox News reported that in Ein el-Hilweh, Lebanon's largest Palestinian refugee camp, revelers fired weapons in the air, with similar celebratory gunfire heard at the Rashidiyeh camp near the southern city of Tyre as well. The Palestinian National Authority (PNA) condemned the attacks and attempted to censure and discredit broadcasts and other news reports. with many newspapers, magazines, Web sites and wire services running photographs. The PNA claimed such celebrations were not representative of the sentiments of the Palestinian people, and the Information Minister Yasser Abed Rabbo said the PNA would not allow "a few kids" to "smear the real face of the Palestinians". In an attempt to quash further reporting, Ahmed Abdel Rahman, Arafat's Cabinet secretary, said the Palestinian Authority could not "guarantee the life" of an Associated Press (AP) cameraman if footage he filmed of post-9/11 celebrations in Nablus was broadcast. Rahman's statement prompted a formal protest from the AP bureau chief, Dan Perry.
Annette Krüger Spitta of the ARD's (German public broadcasting) TV magazine Panorama states that footage not aired shows that the street surrounding the celebration in Jerusalem is quiet. Furthermore, she states that a man in a white T-shirt incited the children and gathered people together for the shot. The Panorama report, dated September 20, 2001, quotes Communications Professor Martin Löffelholz explaining that in the images one sees jubilant Palestinian children and several adults but there is no indication that their pleasure is related to the attack. The woman seen cheering (Nawal Abdel Fatah) stated afterwards that she was offered cake if she celebrated on camera, and was frightened when she saw the pictures on television afterward.
There was also rumour that the footage of some Palestinians celebrating the attacks was stock footage of Palestinian reactions to the Iraqi invasion of Kuwait in 1990. This rumour was proven false shortly afterwards, and CNN issued a statement to that effect. A poll conducted by the Fafo Foundation in the Palestinian Authority in 2005 found that 65% of respondents supported the September 11 attacks.
Rest of the world
People's Republic of China: Chinese President Jiang Zemin said he was "shocked" and sent his condolences to President Bush, while the Foreign Ministry said China "opposed all manner" of terrorism. In Beijing, tens of thousands of people visited the U.S. Embassy, leaving flowers, cards, funeral wreaths and hand-written notes of condolence on the sidewalk out front.
Cuba: The Cuban government expressed its pain and solidarity with its longtime adversary and offered air and medical facilities to help.
Brazil: In Brazil, Rio de Janeiro put up billboards that showed the city’s famous Christ the Redeemer statue embracing the New York City skyline, though it wasn't long before the billboards were defaced with the slogan "The U.S. is the enemy of peace".
India: India declared high alert across most of its major cities and conveyed "deepest sympathies" to the U.S. and condemned the attacks. Children in the country taped up signs that read, "This is an attack on all of us".
Japan: Japanese Prime Minister Junichiro Koizumi expressed "great anger" and said that "these acts of terrorism should not be forgiven." Special security precautions were ordered at all United States military installations.
North Korea: A spokesperson for the North Korean Foreign Ministry was quoted by state-run news agency KCNA as saying: "The very regretful and tragic incident reminds it once again of the gravity of terrorism. As a UN member the DPRK is opposed to all forms of terrorism and whatever support to it and this stance will remain unchanged."
Philippines: Philippine President Gloria Macapagal-Arroyo sent a letter to President Bush assuring the safety of U.S. facilities in the Philippines. She said that "nothing can describe the shock and horror of all humanity in the face of the unimaginable acts of terror inflicted on the United States." She added that the Filipino people extends condolences to all victims of the attacks. Arroyo also ordered the Philippine consulate in New York to search and confirm Filipino casualties of the attacks.
Russia: Russian troops were put on alert in response to the attacks. President Vladimir Putin held an emergency meeting of security officials and said he supported a tough response to these "barbaric acts". He also informed Condoleezza Rice by telephone that any and all pre-existing hostility between the two countries would be put aside while America dealt with the tragedy. In Moscow, women who spoke no English and had never been to America were captured on film sobbing in front of a makeshift tribute on a sidewalk. In addition, television and radio stations went silent to commemorate the dead.
Kazakhstan: Kazakhstan offered the use of its airspace and offered its condolences.
Maasai: The Maasai people gave 14 cows to help and support the United States after the attacks.