Mourners are congregating at the US Embassy in London
The Queen has ordered a special Changing of the Guard ceremony at Buckingham Palace on Thursday in honour of those who perished in Tuesday's atrocities.
And a marquee is being opened at the US Embassy in Grosvenor Square, London, which will house books of remembrance and include a place for floral tributes.
All major towns and cities in the UK have opened books of condolence for people to sign and flags have been lowered to half mast.
The special Changing of the Guard will be held at 1130BST and in an unprecedented move the US National Anthem will be played. It will be followed by a two-minute silence.
The Queen will be represented by the Duke of York and the ceremony will also be attended by the US ambassador in London, William Farish.
The embassy has become a focus in the capital for Americans and Britons who want to pay their respects to those killed.
Many bouquets have already been left and an American flag is draped around a nearby tree.
Paying his respects, Andrew McGee, 32, from London, said: "The attack was a tragedy. I love Americans and when I saw the pictures I just could not believe it."
'Horrified'
John Hall, a 43-year-old computer programmer, originally from Washington state, said the terror attack was a "crime against humanity".
"Everybody was just horrified. A lot of us are also very angry.
"Americans are good people. They don't deserve this and New York is a wonderful city. I think they will build a gigantic park as a memorial in New York."
The Prince of Wales was the first person to sign a book of condolence at the embassy after he called in on the way back from his visit to Somerset on Wednesday.
He wrote his name on the volume in a gesture of sympathy and solidarity with the thousands of victims of the atrocities.
Film withdrawn
After signing the book, the Prince spent time talking to staff, many of whom know people affected by the tragedy.
A US embassy spokesman said: "It was a very simple, solemn occasion. He was joined by Ambassador William Farish and his wife. There was a signing ceremony and a formal handshake."
Residents in the Scottish town of Lockerbie have sent a message of condolence to the mayors of New York and Washington DC.
The town was devastated in 1988 when the Pan Am 103 flight exploded over it after terrorists planted a bomb on the plane.
The letter said: "We received comfort and support from America following the Lockerbie air disaster in December 1988 and we want to let you know you are in our thoughts now and in the difficult times ahead."
In the aftermath of the attack the action movie Swordfish - in which a city block is destroyed by a bomb - has been withdrawn from cinemas nationwide by its distributor Warner Brothers.
The film, starring John Travolta and Vinnie Jones, had been showing in more than 100 cinemas around the UK.
Warner Brothers, which had offices in the World Trade Center, has decided to pull the film over here.
In the film a bomb is strapped to hostages and detonated, obliterating part of west Los Angeles. Several police officers are killed in the blast.