Thousands of people, including the families of victims, wore black or yellow ribbons, symbolizing mourning and hope.

Wiping back tears, they carried flowers and pictures of those who died on the site of the most devastating terrorist assault in U.S. history.

" Today, again, we are a city that mourns," said New York Mayor Michael Bloomberg, setting the tone of the anniversary. "We come here to honor those that we lost and to remember this day with sorrow. But we also remember with pride, and from that comes our resolve to go forward."

One of the most poignant aspects of the memorial service came when more than 200 children who lost relatives in the tragedy approached a microphone in pairs and read the names of those killed in the attack.

Children read names of victims during ceremony at site of World Trade Center attacks

" When my mom told me that I could read names, I just wanted to, because I knew my dad would be proud of me, if I did," said Emily Tompset, who lost her father and participated in the ceremony. "And I also wanted the people that did this to us to know that they could take my dad away, but they couldn't take his spirit away."

At the White House, President Bush paused with his staff on the South Lawn and bowed his head in silence at the time when the first hijacked plane hit the World Trade Center.

Earlier, after attending a church service, the president described the anniversary as a day to remember lives lost and the heroic deeds of those who helped the survivors.

Families holding up victims' pictures during ceremony in New York

" We remember the compassion and the decency of our fellow citizens on that terrible day," he said. "Also, today is a day of prayer. We pray for the husbands and wives and moms and dads and sons and daughters and loved ones of those who still grieve and hurt."

At Arlington National Cemetery outside Washington, Defense Secretary Donald Rumsfeld and General Richard Myers, the chairman of the Joint Chiefs of Staff, placed a wreath near a marker memorializing the more than 180 people killed when a third hijacked airliner hit the Pentagon.

General Myers said all who died that day are heroes, as are those soldiers who have died in the war on terrorism that followed.

"For the last two years, we have been a nation at war," reminded General Myers. "Terrorists are trying to defeat what we Americans stand for - for peace, freedom, tolerance and respect for human life. So we have undertaken an enormous effort to prevent them from spreading their creed of bloodshed, of hatred, of intolerance."

In a dramatic reminder of the continuing danger of terrorism, the U.S. State Department issued a new worldwide alert, warning that al-Qaida my be planning new operations "more devastating" than the attacks two years ago.

The advisory warned of possible attacks overseas, and said U.S. officials "cannot rule out the potential" for the terrorist group to attempt another catastrophic assault on the United States.

The warning came as Secretary of State Colin Powell attended a memorial ceremony, saying the United States is leading nations "around the globe to come together in a worldwide effort to wipe terrorism from the face of the earth."

"Faithful friends and former foes alike have united against terror, and we are bringing every tool of statecraft to bear against it," said Mr. Powell.Ceremonies were also held near Shanksville, Pennsylvania, where a fourth airliner crashed, after passengers rose up to fight their hijackers. All 40 passengers and crew on board died in the crash.