America grieved the victims of Sept. 11, 2001 Sunday, as friends and family members gathered for remembrances at Ground Zero (search) in New York, Arlington National Cemetery in Washington, D.C., and Stoneycreek in Pennsylvania — the three sites of the terrible plane crashes, four years ago to the day.

In New York, family members gathered at the site of the World Trade Center tragedy to read the names of lost loved ones who perished in the attacks and observed four separate moments of silence to mark the time when each jetliner struck the towers and then when each tower fell.

In Washington, D.C., President Bush also observed a moment of silence while standing on the South Lawn, and a wreath-laying ceremony at Arlington Cemetery began with the singing of the “Star-Spangled Banner” as many gathered to remember victims.

In Pennsylvania, Attorney General Alberto Gonzales, former Pennsylvania Governor and Secretary of Homeland Security Tom Ridge, Senator Rick Santorum and Pennsylvania Governor Ed Rendell were to speak at memorial in Stonycreek for the victims of Flight 93.

Chairman of the Joint Chiefs of Staff General Richard Myers spoke at Arlington National Cemetery ahead of a wreath-laying ceremony at which "Taps" was played, as family, friends and coworkers gathered to remember those who lost their lives at the Pentagon and on Flight 77.

"Today we remember our troops in harm’s way … and all those who have served America throughout its history," he said.

“These patriots will not be forgotten,” Myers said.

Speaking at the ceremony, Secretary of Defense Donald Rumsfeld reflected on the day “terrorists struck at the heart of a free people,” killing thousands.

His voice breaking, Rumsfeld noted that the children of some of the 184 victims killed inside the Pentagon and aboard hijacked American Airlines Flight 77 on Sept. 11 were in his audience.

“… perhaps we can tell them this: Throughout human history there have been those who seek power through fear and mass murder but eventually all of them -- every one -- has fallen."

Of the victims, he said, "What united them all is that they had hopes and dreams that were their own and the right and opportunity to pursue them. ...”

“Americans displayed the resolute courage that has defined our country through generations,” he said.

“You are in our thoughts and prayers. May God bless the United States of America.”

Following the ceremony, throngs of people marched in remembrance of the attacks and in honor of troops fighting overseas.

Several thousand participated in the "America Supports You Freedom Walk" organized by the Defense Department to commemorate the anniversary and pay tribute to U.S. troops.

Marchers observed a moment of silence outside the Pentagon and softly sang "God Bless America" before going past the rebuilt wing of the Defense Department headquarters that was hit by the jet. People saw one old brick embedded in the new construction, charred black from the burning jet fuel and etched with the date of the attack.

Marchers made their way toward the National Mall, site of an afternoon concert by country singer Clint Black.

Gordon England, the acting deputy defense secretary, noted the crystal clear day in remarks outside the Pentagon. "But I also remember four years ago was a beautiful day," he said, "and of course it turned into a very dark and fateful day for the world."

In New York, the roll of the lost began with Gordon M. Aamoth Jr., an investment bank employee. Then, one after another, the names began to echo across the site where the World Trade Center (search) towers collapsed four years ago in a nightmarish cloud of dust and debris.

Three hundred twenty pairs of siblings were to read the names of the dead as hundreds of relatives, friends and colleagues watched in pained silence, some holding aloft portraits of their loved ones. Several in the audience broke down in tears.

"We love you. We miss you," read one sign for victim Nereida De Jesus.

As the names were read, weeping mourners filed down a ramp to a reflecting memorial pool at the floor of the 16-acre site, which remains virtually empty four years after the attack killed 2,749 people and tore a hole in the New York skyline. Families dropped red, orange and yellow roses in the still water, some shaking as they inscribed dedications on the wooden edge of the pool.

Houses of worship were to toll their bells throughout the city shortly after a moment of silence at 8:46 a.m., the time at which a hijacked jetliner crashed into the north tower. Three other moments of silence were planned: at 9:03 a.m., marking the moment a second plane struck the south tower, and at 9:59 a.m. and 10:29 a.m., the precise times when each tower collapsed.

Two light beams inspired by the twin towers were to shoot skyward Sunday night in an echo of the towers' silhouette. The "Tribute in Light" will fade away at dawn on Monday.

"Today, again, we are a city that meets in sadness," Mayor Michael Bloomberg (search) said at the start of the ceremony.

He opened with words of condolence for those devastated by the terrorist bombings in the London Underground, and for the thousands of people devastated by Katrina.

"Today, as we recite the names of those we lost, our hearts turn as well toward London, our sister city, remembering those she has just lost as well," Bloomberg said. "And to Americans suffering in the aftermath of Hurricane Katrina, our deepest sympathies go out to you this day."

In New Orleans, firefighters from New York helping with the relief effort gathered around a makeshift memorial for their fallen comrades. At the service, a bell from a neighboring church, its steeple wiped out by Katrina, was given to the New York firefighters.

Earlier in the day, Bloomberg described the mood to FOX News as “remorse combined with optimism" and said of the rebuilding effort: "We have a long ways to go."

But rebuilding efforts should not be rushed, as “history will judge us based on how well we do it, not how fast,” said Bloomberg, adding that the city has devoted 1,000 police officers to combating terrorism since the attacks.

Former Mayor Rudy Giuliani said: "The city is stronger than it was before Sept. 11, 2001," but added: “The memories and the sense of loss are always with you.”

FOX News' Heather Scroope and the Associated Press contributed to this report.