NEW YORK – In what figures to be a somberly stirring ceremony, New York – and, a nation – will once again mark one of the darkest days in U.S. history Tuesday, as thousands gather for Sept. 11 memorial services at Ground Zero in Manhattan.

Marking the sixth anniversary of the terrorist attacks that toppled the World Trade Center’s Twin Towers and killed 2,750 people, the commemoration service in lower Manhattan is scheduled to begin at 8:40 a.m. EDT and last almost four hours.

There will be four moments of silence – two marking when hijacked airliners struck the towers and two when the towers fell on the morning of Sept. 11, 2001.

Throughout the ceremony, emergency responders will read the names of World Trade Center victims over background music.

New York Gov. Eliot Spitzer, former state Gov. George Pataki, New Jersey Gov. Jon Corzine, New York City Mayor Michael Bloomberg and former Mayor Rudy Giuliani, now a presidential candidate, are scheduled to speak at the commemoration.

The observance will take place for the first time at Zuccotti Park, across from Ground Zero.

The city, state and Port Authority had said that unlike past years, Ground Zero was unsafe for a ceremony this year because the reconstruction effort is now well under way.

However, during the ceremony, families of 9-11 victims will be allowed to visit the site where the towers once stood.

Families will begin arriving at 7 a.m. and the ceremonies will officially begin with bagpipers and drummers leading the World Trade Center flag to a stage.

Members of the Brooklyn Youth Chorus are scheduled to perform “The Star-Spangled Banner.”

Bloomberg will open the ceremony by calling for a moment of silence – the first at 8:46 a.m. in observance of the time the first plane struck the north tower – as bells toll for the attack victims in houses of worship throughout the city, officials said.

The names of those killed in the attacks will be read by 118 pairs – or 236 readers, each reading approximately 12 names of those killed.

A second moment of silence will take place at 9:30 a.m., the time the second hijacked plane struck the south tower.

The fall of the south tower will be observed with a moment of silence at 9:59 a.m.

The fall of the north tower will be marked by a moment of silence at 10:29 a.m., officials said.

A flautist, Emily Thomas, is scheduled to perform “Amazing Grace.”

The Brooklyn Youth Chorus also will perform a rendition of “Bridge Over Troubled Water.” Three trumpeters – one member each from the New York City Police Department, New York City Fire Department and Port Authority Police – will play taps.

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AP-NY-09-10-07 1936EDT