President Bush will lead a moment of silence at the White House in honor of the 3,000 people who died Sept. 11, 2001, in the worst terrorist attack on American soil.

The ceremony on the South Lawn will occur at 8:46 a.m., the time that terrorists flew the first commercial jetliner into one of the Twin Towers of World Trade Center. The second plane crashed into the other tower at 9:03 a.m.

Bush will then head to the Pentagon in Washington to dedicate a memorial to each life lost when American Airlines Flight 77 hit that building — a symbol of American military prowess — in the third strike that morning.

The Pentagon monument consists of 184 benches, one for each victim, that overlook small reflecting pools. The Defense Department's headquarters were struck about an hour after the attacks in New York.

A fourth plane that was apparently headed for the White House or the Capitol building in Washington crashed prematurely in a field in Shanksville, Pa.; a ceremony will also be held there.

Joining the president will be first lady Laura Bush, Vice President Dick Cheney and his wife Lynne, members of Congress, Cabinet members, military officials and about 3,000 White House employees and guests.

The Pentagon ceremony will include a wreath laying, music and a reading of the names of those who died on the plane and inside the building. Defense Secretary Robert Gates is expected to speak.

"The president thinks about 9/11 every single day when he wakes up and before he goes to bed," White House press secretary Dana Perino said Wednesday. "This is what he's concerned about. He's always been concerned about another attack on our country. Thankfully, we haven't had one."

Shortly before 9 a.m., families of Sept. 11 victims will go down seven stories into the cavern where the towers stood — called Ground Zero ever since this day in 2001 — using the construction ramp on the south side of the 16-acre site to touch the place where their loved ones died before returning to street level.

Barack Obama and John McCain, the Democratic and Republican presidential nominees, will appear together at Ground Zero in New York on Thursday to honor those who died.

The campaigns agreed to halt television advertising critical of each other for the day.

In New York, four moments of silence were planned Thursday morning to commemorate the precise times that two hijacked jetliners crashed into the World Trade Center, along with the times that each tower collapsed.

Family members and students representing more than 90 countries that lost citizens on Sept. 11, 2001, were to read the names of the more than 2,700 people killed in New York.

Some mourners wondered if the remembrance would, or should, continue as it has indefinitely. About 3,500 people attended last year's ceremony, a roughly 25 percent decrease from 2006.

"We've kept it alive, and perhaps kept it alive too long," said Charles Wolf, whose wife, Katherine, was killed at the World Trade Center. "How many times do you reopen the wounds?"

Other victims' relatives worry that Sept. 11 will revert to being just another date on the calendar.

"The remembrances have to continue; for how long, I can't say," said Barbara Minervino of Middletown, N.J., whose husband, Louis, died in the towers.

The ceremony was to include the reading of 2,751 victims' names, one more than last year. The city restored Sneha Philip, a woman who vanished on Sept. 10, to its official death toll this year after a court ruled that she was likely killed at the trade center.

Former Mayor Rudy Giuliani was scheduled to speak at the ceremony, as he has every year, along with officials including Mayor Michael Bloomberg and Homeland Security Secretary Michael Chertoff.

McCain also was scheduled to attend the service in Shanksville for the 40 people killed there aboard United Airlines Flight 93.

Bush announced this week that he was sending a Marine battalion to Afghanistan in November to replace two that are scheduled to leave, as well as an Army brigade by January.

U.S. commanders in Afghanistan say they need another 10,000 troops — about three times as many as they will receive this winter under the troop deployment plan Bush announced.

The commanders also are pressing for more non-military aid and say the Afghan government must perform better.

The Pentagon Memorial was built at a cost of $22 million on a 1.9-acre parcel of land adjacent to the Defense Department building and within view of the crash site.

The Associated Press contributed to this report.