A silent crowd of thousands looked on as a simple ceremony brought recovery operations at the New York World Trade Center to an end.

The ceremony began with the ringing of the New York Fire Department bell to commemorate the 343 firefighters who died when the Twin Towers collapsed after the terrorist attacks on 11 September 2001.

It was tough to come here every day and now it's tough to leave

Firefighter John Keating

Then a guard of honour of 10 members of the city services slowly carried an empty flag-draped stretcher out of the pit which marks the World Trade Center's foundations.

The stretcher was intended to symbolise the almost 1800 victims whose remains have not been found.

The ceremony began at 1029 local time (1429GMT), the exact time when the second of the Twin Towers fell after being hit by a hijacked plane.

The stretcher was loaded into a New York Fire Department ambulance, which drove away from the site to the sound of drums from a pipe band.

Steel beam

Soon afterwards, a truck followed up the steep incline from the basement of the obliterated Trade Center.

The former mayor and the current mayor attended

It carried the last 30-foot (nine metre) steel beam from the Trade Center that remained standing until cut down on Tuesday evening.

Five helicopters from the NYPD flew over the site, and the pipe drum band played "America the Beautiful" to mark the official end of the recovery operation.

A large banner on the side of the site read "We Will Never Forget", and the crowd threw flowers down into the gaping hole and applauded as an honour guard left the site.

In figures

2,823 people were killed 19,500 body parts collected 1,092 victims identified An additional 1,616 death certificates issued without a body at the request of victims' families 105 people still missing 1,642,698 tons of debris were removed 190,568 tons steel 108,342 truckloads More than 3.1 million hours of labour

"I have been here since Day One. I think we did the best we could do. I would do it again 100 times," said one of the honour guard, crane engineer Rich Streeter.

The nearby Wall Street Stock Exchange observed a two-minute silence in honour of the dead.

More than 2,800 people were killed in the Trade Center attacks, but the remains of only a third of them have been found.

Building the future

New York Mayor Michael Bloomberg who was one of the dignitaries present, said earlier that the ceremony had two aims.

Nearly 3,000 were killed in the attack

"It means that we cannot forget why we're here - 2,800 people gave their lives for freedom, for our right to go about and practice our religion and say what we want to say, and we just cannot forget them," he said on NBC television.

"It is also the point where we have to look to the future and start focusing on building a new world for the relatives that they left behind, and make sure the terrorists know that they can't beat us," Mayor Bloomberg added.

Families' concerns

Victims' families have planned their own service near the site on Sunday to accommodate those who could not attend the ceremony during the working week.

The cleanup operation, involving the removal of 1.6 million tonnes of debris, has been completed ahead of schedule.

We have to look to the future and start focusing on building a new world for the relatives they left behind

Michael Bloomberg Mayor of New York

It has cost $800m and has taken three months less than at first expected, because work has continued around the clock, stopping only for a few memorial services.

The BBC's Jane Standley in New York says the speed of the operation has taken some victims' families by surprise, and many are concerned that they will never get their loved ones' remains identified.

"To not have anything recovered, it's just such an empty feeling," said Jennifer Tarantino, whose husband died on 11 September.

"It's so final. Your husband goes to work one day and that's it, you never see him again."

There is also a growing debate as to what to do with the site.

Many families of victims want it all to be turned into a memorial park, but other plans involve building on almost the whole area.