Some families are unhappy at the new memorial

The structure at the site of the destroyed World Trade Centre will feature two reflecting pools surrounded by trees.

Titled Reflecting Absence, it will bear the names of the 2,900 people who died in the attacks.

The design is opposed by relatives of some of the victims, who argue that it is inappropriate and unsafe.

On Monday, workers arrived to begin work on the memorial, which is due to open in 2009.

Trucks rolled down ramps loaded with timber, and construction workers cleaned the memorial "footprint" of debris and checked for any signs of damage.

New York governor George Pataki called the event "a very important milestone" last week.

Deadline

The memorial and the underground museum face complaints from some relatives of those who died.

REFLECTING ABSENCE Winning design selected from 5,201 submissions Aim is "a space that resonates with the feelings of loss and absence" Two voids on WTC tower footprints contain large pools recessed below ground level Ramps give access to memorial spaces around pools, where names of dead engraved Shaft at western edge of site exposes some of original foundations Staircase in shaft gives access to artefact displays, exhibition area and library Design also includes space for memorial services and room for unidentified remains WTC site plan

They say the design is inappropriate and unsafe. A protest rally was held at Ground Zero on Monday.

"They are the last scar that 9/11 has made on this soil," said Anthony Gardner, director of the Coalition of 9/11 Families.

"One hundred years from now, they'll resonate more with visitors than any memorial or any museum we build on that site," he added.

But New York Mayor Michael Bloomberg said he was opposed to any further obstructions to the memorial.

"I think the time for expressing yourself has come and gone. And now we should rally behind this design and go ahead and support this memorial," he said.

The president of the memorial foundation says while she respects the feelings of those who object, family members were consulted and it was time to begin work.

The 16-acre (6.5-hectare) site is beset with difficulties.

Tuesday is the deadline for the Port Authority of New York and New Jersey, which owns the site, and the developers to resolve their differences over when the office space gets built.

Next month work is due to begin on the Freedom Tower, the skyscraper intended to fill the void in Lower Manhattan's skyline.