US:Relatives of those who died in the September 11th, 2001, terror attacks will remember their loved ones as they descend a ramp into Ground Zero today.
Family members will walk in single file to the lowest level of the site where they can lay flowers and pay their respects.
The procession was a compromise move following safety fears over the use of the site in Lower Manhattan.
The ceremony will pause four times for silence, twice to mark the time when each aircraft hit the twin towers, and twice at the time when each tower fell.
Those who responded to the attacks and helped with the recovery efforts will read the names of the victims.
As the anniversary approached, al-Qaeda leader Osama bin Laden urged the American people to "embrace Islam" in his first video for almost three years.
For the first time, one of New York's television stations originally opted not to broadcast the reading of the 2,749 names killed in the attacks.
However W-ABC TV later changed its decision not to screen the reading of the names, often the most emotional part of the service, following the reaction of victims' families.
Lee Ielpi, president of the September 11th Families Association, choked back tears as he talked about how he lost his firefighter son Jonathan (29), who was married with two children, six years ago.
"No matter how many times you say it, it still has this effect on you," he said. "It stays forever."
His son's ripped jacket and firefighter's helmet - one of just 12 recovered from the debris following the towers' collapse - are on show at the WTC Tribute Centre.
Hundreds of "missing" posters line the centre's walls along with more than 1,200 photographs provided by relatives of their loved ones.
Later today relatives of British victims will attend a separate commemoration at the British Memorial Garden in New York's Hanover Square, about half a mile from the World Trade Center site.
An honour guard of British police officers will fly in for the free community concert and ceremony to commemorate the sixth anniversary.
On September 11th, 2001, a total of 266 passengers and crew were on board the four jets, two of which hit the World Trade Centre towers, while one crashed near the Pentagon in Washington DC. The other jet came down near Pittsburgh.
In a measure of the carnage on the ground, five hours after the attacks there were no estimates of the overall death toll.
Images of people jumping from the towers were sent around the world as the then New York city mayor Rudy Giuliani urged people to be calm and to walk out of Lower Manhattan.
World travel was also thrown into chaos. Many US-bound flights had already left the UK when officials closed all US airports and thousands of passengers were diverted or turned around mid-Atlantic.