Mourners gather to commemorate dark day

Law enforcement authorities in both New York and Washington were on high alert on the 10th anniversary of the September 11 attacks.

Security was stepped up in major cities across the nation after the CIA was warned that al-Qaeda may have sent attackers, some of whom may be US citizens, to bomb one of the cities.

Metal barriers were erected on roads near the World Trade Center, while police in New York and Washington were stopping and searching large vehicles entering bridges and tunnels.

Commemoration ceremonies will take place in New York and Washington.

The Press Associated said President Barack Obama and his predecessor George W. Bush will attend events at Ground Zero in Manhattan. Obama will also visit the other crash sites in Pennsylvania and at the Pentagon.

Moments of silence will be observed to coincide with the exact times that the hijacked planes hit in 2001, starting at 8.46am local time.

Additional periods of reflection will take place at 9.59am and 10.28am to mark the moments that the two towers at the World Trade Centre fell.

Obama is due to speak at the commemorative event in Washington.

The Washington Post reported that an official memorial was earlier erected outside of Shanksville, Pennsylvania, in honor of those who died on United Flight 93.

Thousands attended the dedication of a new national park, one day before the 10th anniversary.

Following the terrorism warning, which officials said was "credible but unconfirmed," Obama earlier met with his national security team. He called for a "heightened state of vigilance and preparedness."

The White House said it's looking to extend its heightened security alert beyond September 11.

Meanwhile, in a letter to New York's archbishop, Timothy Dolan, Pope Benedict XVI on Saturday condemned violence in the name of God.

(GlobalPost reports: Pope marks 9/11 by warning that religion cannot justify violence)