Nearly 3,000 people died in the World Trade Center attacks

The Tribute in Light will be installed in a vacant space next to the wreckage of the World Trade Center.

Two searchlights will send 88 high-powered beams of light vertically upwards to light up the Manhattan skyline, beginning each evening at dusk and continuing until 2300 EST.

The columns of light, designed to evoke the destroyed twin towers, will be one of two temporary memorials to be unveiled on Monday.

The other, The Sphere, is a sculpture of steel and bronze that once stood in the plaza between the twin towers.

The Sphere will be unveiled at the exact time the first plane hit - 0846 EST. It is a battered sculpture, badly damaged during the attacks, but remains structurally intact.

It will now be framed by trees and benches in Battery Park in lower Manhattan as a place for contemplation.

Remembering the dead

These two memorials are the first official tributes to the victims, but both will be temporary until the highly controversial decision is taken about what to do with the site - to keep it all as a memorial or to rebuild it.

I want to stress that both of these are temporary, but they give us a place to go and to reflect and to remember and to pray

Mayor Michael Bloomberg

They will highlight the six months which have passed since the attack, but they will also highlight the sensitive issue of how to remember the dead.

The light column was originally to have been called Towers of Light, but protests from victims' families forced a name change. They felt Tribute in Light better reflected the lives rather than the property lost.

Mayor Michael Bloomberg emphasised the importance of the memorials, saying: "I want to stress that both of these are temporary, but they give us a place to go and to reflect and to remember and to pray."

But with the site expected to be cleared by May, pressure is growing for a decision on what it will become.

Families largely want the land left as a memorial park. But business leaders are already campaigning for rebuilding, to boost the local economy.