World leaders have declared war on terrorism following the carnage unleashed in the United States, while some in the Muslim world have openly cheered the loss of American lives.
Memorial services and marches of solidarity were held in some places, while many US consulates worldwide were festooned with flowers and notes of sympathy.
In the biggest show of solidarity, the NATO allies offered Washington unprecedented military support in the battle against international terrorism, invoking a collective defence clause which deems an attack on one alliance member to be an attack on them all.
"The council agreed that, if it is determined that this attack was directed from abroad against the United States, it shall be regarded as an action covered by Article Five of the Washington Treaty, which states that an armed attack against one or more of the allies in Europe or North America shall be considered an attack against them all," a NATO statement said.
"What the allies are saying is what they did was unacceptable, barbaric, and we stand today in solidarity with the United States," said NATO Secretary General George Robertson after the unanimous decision of the North Atlantic Council in Brussels.
"What happened in the United States yesterday could have happened to any of the other 18 members of the alliance," he added.
The European Union vowed to help the United States identify and "punish" those behind terrorist attacks in New York and Washington.
"We will spare no effort to help identify, bring to justice and punish those responsible," EU foreign ministers said in a strongly-worded statement after an emergency meeting in Brussels.
"There will be no safe haven for terrorists and their sponsors."
However, not all the international reaction was sympathetic.
Iraqi President Saddam Hussein declared that the United States had "reaped the thorns planted by its rulers" when New York and Washington came under terrorist attack.
In Nigeria, the world's largest black country, President Olusegun Obasanjo sent "heartfelt condolences" to the US people, but northern Muslims openly celebrated Tuesday's terror attacks.
"When I heard the news, I decided to take the day off to celebrate," 31-year-old Saminu Abdullahi, a motorcycle taxi driver, told AFP, sitting sipping tea in the main market in Kano, the largest city in mainly Muslim northern Nigeria.
Jordan's King Abdullah II said the attacks in the United States would not have taken place if the Middle East problem had been resolved.
"If the United States had resolved the problems in the Middle East, notably the Israeli-Palestinian question, I seriously doubt that they (the attacks) would have taken place," the king said in an interview with the US Cable News Network (CNN) television channel.
Egypt's Foreign Minister Ahmed Maher warned on Wednesday against "anticipating" the US terrorist attack investigations, after Israel's defence minister pointed a finger at Islamic extremists.
However Afghanistan's ruling Taliban said it would consider requests for the extradition of the prime suspect in the attacls the Saudi-born extremist Osama bin Laden, depending on the evidence provided.
Scores of well-wishers turned up outside US embassies from South Africa to the Ukraine to express their sympathy following the terrorist attacks in the United States.
"I don't know anyone in America. I just feel so upset and unhappy," Lucille Butow tearfully told reporters in Pretoria.
Some 10,000 people took to the streets of Germany's financial capital Frankfurt in a silent march to show their support for the United States after Tuesday's deadly terrorist attacks.
US President George W. Bush branded the terrorist strikes at the heart of US power "an act of war" and vowed to avenge the murders of thousands by waging a "monumental struggle of good versus evil."
Bush promised to rally the world against an enemy that seized four commercial jetliners and sent them on suicide missions against the US economic powerhouse the World Trade Centre in New York and the Pentagon -- the seat of its military might.
Canadian Prime Minister Jean Chretien said the terrorist attacks in the United States were "the equivalent of war."
The global support and solidarity was not confined to the United States' traditional allies.
Russia put its intelligence service to work on Wednesday in the hunt for those responsible for the devastating US attacks after President Vladimir Putin urged the world to go to war against terrorism.
Ordinary Russians, many of whom laid flowers and lit candles outside the US embassy in Moscow, said they felt vindicated in their view that the ruthlessness of the Russian campaign in Chechnya was a legitimate response to terrorism.
Russia and the European Union plan to step up cooperation to fight international terrorism, the Russian foreign ministry said following talks between officials of the two sides Wednesday.
Chinese President Jiang Zemin discussed the attacks with Bush late Wednesday and expressed sympathy for the US government, people and families of the victims.