Many of them came in tears, clutching their lilies and roses, as a small traffic island in the centre of London became Britain's impromptu shrine to the dead of America.
In front of the United States embassy, where the Stars and Stripes flew at half-mast, dozens avoided the taxis to cross the road and leave their tributes behind crowd barriers, underneath a single maple tree.
Many laid their bouquets and left quickly, dabbing their eyes with handkerchiefs, but others lingered, saying they did not know where else to go.
Americans trying to get home from business trips and holidays said they had been told by embassy officials they could not expect to get flights for a week and had come to seek solace with their compatriots.
But many more were still trying to get information about friends and family who they feared had been working at the World Trade Centre and the Pentagon when the hijacked aircraft struck.
"It's like my back yard," said Kate Hall, 36, from Washington DC, whose work for a defence contractor meant she was often at the Pentagon. "The majority of the people that work on one of our contracts sit there on that side of the Pentagon.
"Quite frankly, I am almost afraid to call. Yesterday, I sat glued to the television and tried to call some people. At least there were no messages on my hotel phone.
"We were in a restaurant having lunch and I looked up at the television set. I thought it was Beirut. That sort of thing doesn't happen in the US. It feels strange being overseas while this is happening in my country."
Ms Hall, who was visiting London with her British fiancé, left a single flower with the message: "To the victims and their families, God speed."
Some carried similar sentiments but another left by a woman from southern Ireland spoke of anger and vengeance. "To the citizens of America. This cold-blooded act of mass murder was meant to bring you to your knees. But stand tall my friends, the people of the free world will strengthen your battlements. We will stand by you shoulder to shoulder. United, we will go get the murdering bastard."
But overwhelmingly, even in Grosvenor Square, this very American corner of London, which is dominated on one side by its embassy, and a statue of Dwight Eisenhower, the emotion was controlled and very English.
Despite a number of police officers in protective tunics around the embassy, the security presence was low key. Police on duty on the pavement said they were on amber alert – one step down from red.
Two officers stood back as the steady stream of mourners came to the tree to lay flowers and in one case, a framed picture of the pre-crash Manhattan skyline with the twin towers prominent. Someone else had tied a yellow ribbon to the tree and later in the morning it was joined by an American flag.
Mary Spillane, 50, an American expatriate, turned up with her dog to leave her tribute to the victims, including friends she believes died in the World Trade Centre. "I phoned one of the offices and I got through to reception before I got cut off," she said as she left in tears.
Some had already received confirmation of what they had previously feared. Lelanie Jacobs, 21, from South Africa, said her flatmate, a computer engineer working on the 90th floor of the north tower, had died. "I didn't know until this morning when I got an e-mail from his friends," she said.
"I can't fully understand it now. Coming down here brings me a little bit closer to everything. I'm shocked, I don't know what to do."
Debbie Macarthur, 28, a New Yorker who teaches in Britain, was told about the attack by a parent of one of her pupils who asked her if she knew if "America was on fire".
Mrs Macarthur, who is six months' pregnant, said: "I have a very close cousin who has an office in the World Trade Centre, but I found out he had a meeting uptown.
"I sat there dialling the phone, dialling the phone. We e-mailed and finally got one back."
In her shock, she was joined by many Britons: City workers with colleagues in the United States, couples comforting each other and an elderly man in a soldier's uniform.
"I served alongside the American forces. I remember in the War they came to our assistance," said Ray Gardiner, 72. "I think it's completely dreadful and I think we should give the Americans every bit of support that we can."Reuse content