Rudolph Giuliani believes the platform will help visitors

"I really urge Americans to come here and say a little prayer and reflect on the whole history of America," said city Mayor Rudolph Giuliani, while standing on the 13ft-high (3.9m) viewing platform in Lower Manhattan.

The platform can hold 400 people at a time and three others are planned.

Mr Giuliani said there were some concerns that the platform would help turn the site into a tourist attraction, but he thought most people would be respectful.

"People from all over the world want to come here, for the most appropriate of reasons," he said.

"It would be like denying people access to Gettysburg or Bunker Hill or Normandy."

Site almost cleared

Mr Giuliani left a message to rescuers on the platform railing which read: "We will always remember what you did here - you our heroes - to save America. God bless you."

From the platform visitors can see the immense area, now almost entirely cleared.

Each person has only a few minutes - enough for a photograph or some silent meditation - before being asked to move on.

Paulette Canuso, 52, who travelled to the site from Philadelphia, said: "It is more of a religious experience than a tourism one. I felt like making a pilgrimage."