Americans will mark the third anniversary of the Sept. 11, 2001, attacks today in the shadow of both the presidential election and continuing fears that al Qaeda is determined to stage another attack to disrupt the U.S. political process.

With both President Bush and his Democratic rival, Sen. John F. Kerry (Mass.), making the fallout from the attacks central components of their election campaigns, today's anniversary will be more politically charged than the previous two.

Bush and his wife, Laura, will attend a prayer service this morning at St. John's Episcopal Church in Lafayette Square. They will later observe a moment of silence on the White House's South Lawn, at 8:46 a.m., the time the first plane struck the World Trade Center.

Kerry will honor victims of the attacks, including those who were on the two planes that took off from Boston's Logan International Airport that day, at the Boston Opera House. He has also recorded a national radio address for the anniversary.

"So much happened this year that can be traced back to 9/11," said Heidi Snow, who runs a counseling service for families of airline disaster victims called AirCraft Casualty Emotional Support Services. "Often, if there is a pressing political event or issue, like we have this year, you'll find that people tend to vent their grief towards that. We've seen a lot of anger."

Although U.S. government officials have played down the likelihood of a terrorist attack around the anniversary, they stressed that they remain deeply concerned that terrorists may be planning an attack to disrupt the Nov. 2 elections.

On Thursday, television networks aired a video in which al Qaeda's second-in-command, Ayman Zawahiri, predicted that the defeat of U.S. forces in Iraq and Afghanistan was a matter of time. This is the third year that the terrorist network has released a tape around Sept. 11.

"We remain concerned that al Qaeda continues to demonstrate a desire and an intent to attack the United States to affect the democratic process," Department of Homeland Security spokesman Brian Roehrkasse said. "But we have no specific evidence a particular plot is in motion."

Law enforcement officials in Washington said that because they are already operating under an orange alert, they will not be taking significantly more precautions around the anniversary.

"Obviously, it is a time that represents a potential threat period for us," said Michael A. Mason, the assistant FBI director in charge of the bureau's Washington field office. "We certainly have asked everyone to raise their level of vigilance, but given that we are at threat level orange, this is a normal posture for us. We have not significantly increased the normal staffing."

But, Mason added, law enforcement across the county "needs no reminder of the significance of 9/11, and as a result I would assume that their readiness posture has probably been increased as a result."

In keeping with the tradition of the past two anniversaries, New York will be the focus of today's commemorative events, with a ceremony at Ground Zero featuring bereaved parents and grandparents reading aloud the names of the 2,749 who were killed there. In previous years, children of the victims performed the readings. Mourners at the site will also observe four minutes of silence to mark the periods when the two planes struck and when the towers collapsed.

For Barry Zellman, whose brother Kenneth died in the World Trade Center attacks, the day has become an annual pilgrimage.

"I will always go to the place where my brother took his last breath," said Zellman, who lives in New Jersey. "It's a sacred burial ground for me. It keeps me connected to him. Sometimes it feels like America has gone past that day in so many ways, but for me there's no question of being anywhere else on September 11."

Firefighters in New York will hold a number of events to honor the 343 active and three retired firefighters who died in the attack, including a memorial Mass at St Patrick's Cathedral.

"It's a reminder of what we went through and what we will have to deal with for the rest of our lives," said Rudy Sanfilippo, a firefighters union official.

In Washington, Defense Secretary Donald H. Rumsfeld will join bereaved families for a wreath-laying ceremony at Arlington National Cemetery to remember 184 people who died when American Airlines Flight 77, which was hijacked after it took off from Washington Dulles International Airport, struck the Pentagon. At Arlington County Courthouse, a bell will toll 184 times in memory of those who died.

Kelly Jackson, whose father, Jimmie I. Holley, was killed in the attack, said she plans to attend the ceremony at Arlington National Cemetery so that she can "pay tribute . . . pay my respects and ease my pain a little." She also intends to visit the grave of her father, a Pentagon accountant from Lanham who was 54 when he died. She will wait for a quiet moment, she said, and pray.

"I want to talk to him and let him know he's still with me," she said.

Memorial and religious services will also take place in Shanksville, Pa., where a fourth hijacked plane, United Flight 93, crashed in a field outside the town, killing 40 passengers and crew members. Bells will be rung across the state to mark the exact time the plane went down.

Staff writers John Mintz, Sari Horwitz, Christian Davenport, Mike Allen and Paul Farhi contributed to this report.