President Barack Obama will lead tributes to the nearly 3,000 people killed on September 11, 2001 on the eighth anniversary of the devastating attacks.
Events were again scheduled across the U.S. to remember the day when Americans watched in horror as four airliners hijacked by Al-Qaeda were flown into the World Trade Center towers in New York, the Defense Department headquarters near Washington and a Pennsylvania field.
Obama will pay tribute to the victims in a speech at the Pentagon, then meet with relatives of those killed in the attacks. Vice President Joseph Biden will attend commemorative events in New York.
The day of tributes begins in New York, where two airliners slammed into the Twin Towers, killing 2,752 people and prompting President George W. Bush to declare a "war on terror". At the site, relatives of those killed will join volunteers from across New York city to read the names of the victims.
The public reading, now an annual ritual, will be paused four times to mark the moments when American Airlines Flight 11 and United Airlines Flight 175 hit the buildings, and when the two towers collapsed.
At nightfall, two beams of light will shoot skyward from the site.
Obama is to lead the tributes in Arlington, Virginia, where a third hijacked plane, American Airlines Flight 77, crashed into the Pentagon building. The president, accompanied by Defense Secretary Robert Gates and chairman of the Joint Chiefs of Staff Admiral Michael Mullen, is to observe a moment of silence, deliver a speech and lay a wreath.
He will then meet with relatives of the victims and tour a memorial to the 184 people killed on the ground and aboard Flight 77.
Russian Prime Minister Vladimir Putin offered condolences for the anniversary, noting his country recently marked five years since the Beslan school hostage crisis in which more than 330 people, mostly children, were killed. "Russia, like no other country, knows what this is like," Putin said in televised remarks.
The Pentagon memorial is the only major official monument to the victims of the September 11 attacks, with plans for similar sites in New York and Pennsylvania held up in part by financial and legal wrangling.
At the World Trade Center site, progress has been slow on the foundation of "Freedom Tower" -- part of a planned complex of five new skyscrapers, with a park and memorial in the middle. With plans hampered by the financial crisis and the real estate downturn, the site just looks like a large hole, although work on foundations of several key elements is underway and the frame for the future tower is rising.
According to a poll last week by Quinnipiac University, 25% of New Yorkers said the slow pace on constructing a memorial made them "ashamed," the highest number to give that answer since it was first asked in 2006.
In Shanksville, Pennsylvania, where United Airlines Flight 93 crashed in an open field after passengers overwhelmed the hijackers, tributes begin with a reading of victims' names. Many believe the hijackers intended to crash the plane into the Congress building in Washington.
Prayer services and interfaith remembrances are scheduled throughout the day, with a candlelight "peace vigil" closing out the commemorations.
Plans for a Flight 93 memorial have been hampered by controversy over the shape of the monument, and arguments over whether the government can seize private land for the site. A final design, featuring a wall with the names of the victims, is scheduled to be completed by September 11, 2011.
On Capitol Hill, lawmakers on Sept. 2 unveiled a bronze plaque paying tribute to "the passengers and crew of Flight 93, whose brave sacrifice... not only saved countless lives but may have saved the U.S. Capitol from destruction."
Copyright Agence France-Presse, 2009