Queen Elizabeth led Britons of all faiths in mourning the victims of the terrorist atrocities in Washington, New York and Philadelphia at a poignant service of remembrance in St Paul's Cathedral, London.
At Westminster, a solemn House of Commons reconvened to hear the newly-elected Conservative leader, Mr Iain Duncan Smith, pledge bipartisan support for the British Prime Minister, Mr Tony Blair, in his determination "to stand shoulder to shoulder" with their friends and allies.
Addressing a congregation of 2,600 in St Paul's, the Archbishop of Canterbury, Dr George Carey, his voice raw with emotion, recalled the "senseless evil" perpetrated against the US and the free world on Tuesday.
Liberty, he said, "must be defended" and those responsible for such "barbaric acts" held to account. But as leaders faced the "awesome responsibility" of deciding how to respond to the terrorist attack, he cautioned against perpetrating further evil aggression: "We must be guided by higher goals than mere revenge."
Earlier, Mr Blair told MPs the tragedy was of "epoch-making" proportions and called on the world to unite against the outrage of terrorism.
"This is a moment when every difference between nations, every divergence of interest, every irritant in our relations, are put to one side in common endeavour. From Russia, China, the EU, from Arab states, from Asia and the Americas, from every continent of the world has come united condemnation. This solidarity should be maintained and translated into support for action," he said.
Mr Blair enjoyed strong support from Mr Iain Duncan Smith and from the leader of the SDLP, Mr John Hume, for his assertion that Islam was not to blame. "If it appears likely it is so-called Islamic fundamentalists, we know they do not speak or act for the vast majority of decent, law-abiding Muslims throughout the world. I say to our Arab and Muslim friends: neither you nor Islam is responsible for this," Mr Blair said.
Several thousand people, many of whom had arrived in the early hours, lined the streets around St Paul's Cathedral to listen to the service, which was relayed on loudspeakers.
Inside the cathedral, the congregation sang the Star Spangled Banner at the beginning of the service.
After the 50-minute service, Queen Elizabeth, the Duke of Edinburgh, Prince Charles and the US ambassador to Britain, Mr William Farish, walked into the chilly September sunshine to speak to a group of about 50 US citizens who had gathered on the cathedral steps.