Americans took part in silent reflection and fresh mourning Monday, five years after the Sept. 11, 2001, terror attacks that killed 2,973 people.

President Bush capped the day by addressing the nation Monday night from the Oval Office after attending tributes at the World Trade Center, Pentagon and crash site of United Flight 93 in Shanksville, Pa.

In his speech, the president honored the heroism of ordinary Americans who responded to the worst terrorist attacks in U.S. history. (Full Story)

"This struggle has been called a clash of civilizations," Bush said. "In truth, it is a struggle for civilization. We are fighting to maintain the way of life enjoyed by free nations."

The president also appealed to Americans to not waver in the face of the ongoing War on Terror.

Transcript: Bush's Sept. 11 Address From the Oval Office

"Winning this war will require the determined efforts of a unified country. So we must put aside our differences, and work together to meet the test that history has given us," Bush read. "We will defeat our enemies ... we will protect our people ... and we will lead the 21st century into a shining age of human liberty."

America Remembers

Solemn tributes and memorials were held in cities and towns across the nation.

"If I could build a staircase to heaven, I would, just so I could quickly run up there to have you back in my arms," Carmen Suarez, widow of city police officer Ramon Suarez, said at the Ground Zero podium while reading names of the 2,749 people who died there five years ago.

Get complete coverage of the fifth anniversary of 9/11 in FOXNews.com's special Sept. 11 Center.

On the 16-acre New York City expanse where the World Trade Center towers once stood, four moments of silence were held at 8:46, 9:03, 9:59 and 10:29 a.m., the times when jetliners struck each of the twin towers, and when each tower fell.

Some spouses and partners of Trade Center victims who read off names included brief personal tributes to their own loved ones.

"Honey, I want you to have a happy grandparents' day in heaven," said Elaine Moccia, addressing her late husband, Frank Moccia Sr, as she released a balloon gently into the sky where the towers once rose 110 stories above the New York skyline.

Families of the victims began arriving before 7 a.m., many clutching pictures of their loved ones, descended the ramp into what's known as "the pit" to roam the area and lay flowers. Some wore pins bearing pictures of the victims. The mournful sound of bagpipes, so familiar from the seemingly endless funerals that followed Sept. 11, echoed across ground zero after a choir performed the national anthem.

"We've come back to remember the valor of those we've lost, those who innocently went to work that day and the brave souls who went in after them," former Mayor Rudolph Giuliani said in a ceremony at Ground Zero.

Photo Essay: Attack on the World Trade Center

Photo Essay: Attack on the Pentagon

Photo Essay: Flight 93 Crashes in Shanksville, Pa.

Photo Essay: The World Mourns

Photo Essay: America Rebuilds

Family members at Ground Zero held up signs reading "You will always be with us" and "Never forget," and quiet sobs could be heard as the moments of silence were observed. Some victims' relatives crossed themselves and wiped away tears.

"Five years have come, and five years have gone, and still we stand together as one," Mayor Michael Bloomberg said. "We come back to this place to remember the heartbreaking anniversary — and each person who died here — those known and unknown to us, whose absence is always with us."

A youth choir sang "America the Beautiful" and "My Country 'Tis of Thee," and religious leaders of several faiths offered words of comfort.

"I think it's important that people remember as years go on," said Diana Kellie, of Acaconda, Mont., whose niece and niece's fiance were killed on one of the planes. "The dead are really not dead until they're forgotten."

'Americans Will Never Bow to Terrorism'

Peter Gorman, president of the New York Uniformed Fire Officers Association, took note of the day's vivid blue sky and said it reminded many of the late-summer morning of Sept. 11, 2001.

"Today is still a glorious day in the glorious city of New York, the powerful state of New York, in the United States of America," Gorman said. "New Yorkers and Americans will never bow to terrorism, thanks to the U.S. military, thanks to every first responder in this country."

Firefighter Tommy King and others stood beside a fire truck with a windshield emblazoned with the names of two comrades who died on Sept. 11.

"It's just weird being back here," King said outside the World Financial Center, where he hasn't been for five years. "This building here was a morgue."

Construction on a Sept. 11 memorial and on the 1,776-foot Freedom Tower began only year.

President Bush opened the day at a historic New York firehouse, mingling with firefighters and police officers who were among the first to rush to the burning skyscrapers. He later laid a wreath on the Pennsylvania field where United Flight 93 crashed and met with families of Pentagon victims in Arlington, Va., where 184 people died.

Bush also planned a prime-time address from the Oval Office.

There were also moments of silence at 8:46 a.m. in the American and United terminals of Logan International Airport in Boston. American Airlines Flight 11 and United Airlines Flight 175 took off from Logan before slamming into the towers. Security screeners stopped checking passengers for a moment and turned to an American flag. Passengers in line joined in the silent tribute.

"It's a difficult moment for everybody," said National Guard Cpl. Christopher Jessop, who joined the Guard on Sept. 12, 2001.

In Shanksville, Pa., the site where United Flight 93 crashed, a ceremony was held with state and federal leaders as well as family and friends of the passengers of that flight, which took off from Newark Liberty International Airport in New Jersey. That ceremony opened with a prayer.

"These men and women stood in solidarity so others would receive salvation," said Tom Ridge, former governor of Pennsylvania and the nation's first homeland security secretary.

Bush and his wife, Laura, met privately with families at the crash site, which was not open to the public. A 10-foot chainlink fence covered with American flags, firefighter helmets and children's drawings stood at the site. Standing without umbrellas in a cold rain, the Bushes bowed their heads for a prayer and the singing of "Amazing Grace."

"One moment, ordinary citizens, and the next, heroes forever," retired Gen. Tommy Franks said, alluding to the Flight 93 passengers. "We mourn their loss, to be sure, but we also celebrate their victory here in the first battle on terrorism."

Many of the visitors, like 15-year-old Carol Fritz, had no connection to the doomed flight.

"I didn't understand when everything happened," Carol said, crying. "My kids, my grandkids are going to ask me what happened. I wanted to tell them, tell them I was here."

Vice President Dick Cheney and his wife, Lynn, attended a service of prayer and remembrance in Washington Monday morning. Former British Prime Minister Margaret Thatcher joined them. Cheney then joined Defense Secretary Donald Rumsfeld at a ceremony at the Pentagon.

Rumsfeld choked up during his brief remarks at that event.

"I remember working our way through that long tragic day," he said, pausing in between sentences, noting that family and friends of those lost wore double American flag lapel patches. "Know that you are always in our thoughts and prayers," he added.

Cheney said, "September 11 ceased to be an ordinary day in America."

"We honor the men, women and children whose lives were taken suddenly and so coldly here at the Pentagon, the World Trade Center and on a field in Pennsylvania," the vice president continued. "We remember all that we saw and heard and felt on that Tuesday morning and how the world changed on the eleventh of September, 2001. Nine-eleven is a day of national unity that stays with all of us … we were meant to take it personally and we still do take it personally."

Camp Blanding, Fla., remembered Florida soldiers killed in action since Sept. 11. Firefighters and law enforcement officers were to be honored at Idaho's Statehouse. And in Muncie, Ind., a service was set at a funeral home that features a Sept. 11 garden with twin glass towers that light up at night.

In Chicago, people filled churches to pray and remember the victims. In Virginia Beach, Va., firefighters and residents formed a human flag. Firefighters in Akron, Ohio, displayed 3,000 American flags on a 10-acre spiritual center.

Around the world, heads bowed at Sept. 11 remembrances.

"Nine-Eleven will be in our memory forever," Rome Mayor Walter Veltroni said at a downtown piazza. "We all remember where we were, what we were doing, what our first reaction was."

German Chancellor Angela Markel warned that "tolerance and respect for other cultures" must be hallmarks of the international fight against terror, and New Zealand Prime Minister Helen Clark said the world was not safer since 2001.

On Sunday, Bush and the first lady set wreaths in small, square reflecting pools in the pit of the World Trade Center site, one each for where the north and south towers stood. Also in attendance were New York Gov. George Pataki, Bloomberg and Giuliani, hailed for his work as mayor in the months after the attack.

The Bushes also attended a memorial service at St. Paul's Chapel near Ground Zero, where George Washington once prayed and where exhausted rescuers sought refuge in 2001 while they dug through the trade center rubble.

The Associated Press contributed to this report.

Complete coverage of the anniversary of 9/11 is available in FOXNews.com's special Sept. 11 Center

Click below for FOXNews.com original stories about heroes lost on Sept. 11 and how their families are coping with the loss:

World Trade Center: Family of Sept. 11's 'Man in the Red Bandanna' Remembers Their Hero

World Trade Center: Time Ebbs Away at the Grief for a Sept. 11 NYPD Widow

Shanksville, Pa.: Sept. 11, Flight 93 Forever Changed Husband's Life

Pentagon: A Larger-Than-Life Father, a Larger-Than-Life Loss