Memorial service was held Sunday at the National September 11 Memorial, site of the destroyed World Trade Center twin towers, to mark the 10th anniversary of the Sept. 11 terror attacks in 2001, amid tightened security.

People gather for a commemoration at Ground Zero in New York, the United States, Sept. 11, 2011. New York marks the 10th anniversary of the September 11 attacks on Sunday. (Xinhua/Zhu Wei)

New York state and city officials joined family members of victims of the 2001 and 1993 attacks at the reflecting pools, which were built over the towers' footprints.

The ceremony, which is expected to last more than four hours, will pause six times -- twice to mark the times when each of the twin towers was hit, twice to mark the time when each tower fell. The other two pauses are to mark the times when the Pentagon was hit and when Flight 93 crashed in Shanksville, Pennsylvania.

Following the first moment of silence, Obama read passages from the Bible. Then families of the victims started reading of the names, which were carved into stone at the reflecting pools.

Obama is scheduled to be present at ceremonies in Shanksville and at the Pentagon later Sunday.

Bush read a letter by Abraham Lincoln, to a mother, whose five sons were killed in battles, after the second pause.

The ceremony was held amid heightened security across the city. In addition to security measures in and around the site of memorial service, police have established checkpoints at major transit hubs, bridges, tunnels and landmarks.

There were earlier reports about intelligence that al-Qaeda may have sent three operatives into the United States to detonate a car or truck bomb in New York or Washington, D.C.

VietNamNet/Xinhuanet