The hate-filled men who committed these acts debased mankind

William Farish, US ambassador

Up to 300 Britons are believed missing as a result of the attacks on the World Trade Center and Pentagon on 11 September.

Dignitaries standing among the hundreds of people in front of the US embassy included the head of the diplomatic service Sir John Kerr, and Culture Secretary Tessa Jowell.

The Royal Marine band played the US and British national anthems and a lone piper played a verse of Amazing Grace.

'Brave families'

Speaking during the ceremony, Mr Farish said: "Fourteen days ago, at almost this very moment, we watched thousands of people murdered before our very eyes.

"We were shocked and stunned, disbelieving, we thought it couldn't be real, but it was.

Grosvenor Square has become a focus for mourners

"Within hours we heard stories of heroes, firefighters and police officers who stepped willingly into harm's way to give their lives to save others.

"Passengers in an airliner, knowing they would die, said goodbye to their loved ones and then rushed the hijackers.

"Their courage helped us overcome our despair."

Mr Farish paid tribute to the British families who had lost loved ones.

"Words can't describe what these terrific brave families have gone through," he said."

"They are just off the plane from New York. They've all lost sons, daughters and relatives."

Shared grief

He said the floral tributes left in Grosvenor Square since the tragedy had transformed it from "a garden of sadness to one of solidarity".

"The people of Great Britain and the United States have mourned together, now we stand together to face a new challenge. Together we will prevail."

Sir John Kerr said he was present not as the head of the foreign service, nor as a former British Ambassador to Washington, but as a "family friend".

"America's grief is our grief," he said. "Our family lived in America, my children went to the neighbourhood school, played by the Potomac, went to the top of the World Trade Center, and came to love America, from sea to shining sea, and we always will."

Ms Jowell said that two weeks after the tragedy, the power of words to express grief, sorrow and outrage was almost exhausted.

"But every single one of the bunches of flowers laid here represents a shared grief, shared outrage, and solidarity and friendship, felt by people up and down the country as they made a pilgrimage to here in London in order to feel at one with their friends in America," she said.