Sirens wailed through San Francisco before dawn Tuesday to mark the moment 100 years earlier when the Great Quake struck, shattering the city and touching off fires that burned for days.

Eleven centenarians who survived the devastation were joined by thousands of spectators for a memorial ceremony to remember one of the worst natural disasters in U.S. history.

The annual wreath-laying at Lotta's Fountain, the bronze-colored downtown landmark where San Franciscans gathered in the aftermath, was billed as the biggest ever and a tribute to the city's resilience.

"What an extraordinary example: the pioneering spirit that defines our past, I would argue defines our present, and gives me optimism of the future," said Mayor Gavin Newsom. "San Francisco, a city of dreamers. And San Francisco, a city of doers."

The ground held steady during a moment of silence as a bell tolled nine times. The calm was shattered by the cry of sirens that moaned in tribute as three horse-drawn firetrucks arrived.

Most of the city's 400,000 residents were still in bed when the magnitude-7.8 earthquake struck at 5:12 a.m. on April 18, 1906.

The foreshock sent people scrambling, and the main shock arrived with such fury that it flattened crowded rooming houses.

The epicenter was a few miles offshore of the city, but it was felt as far away as Oregon and Nevada. In 28 seconds, it brought down the City Hall it had taken 27 years to build.

From cracked chimneys, broken gas lines and toppled chemical tanks, fires broke out and swept across the city, burning for days. Ruptured water pipes left firefighters helpless, while families carrying what they could fled the advancing flames to parks that had become makeshift morgues.

Historians say city officials, eager to bring people and commerce back to the city, radically underestimated the death toll. Researchers are still trying to settle on a number, but reliable estimates put the loss above 3,000, and possibly as high as 6,000.

The centennial was both a somber remembrance and a celebration of the city's ability to rise from the ashes.

"It doesn't really feel like a party to me," said Bob McMillan, 37, who walked to the event with his wife and 2-year-old daughter. "There is a sense of the tragedy but there is also that San Francisco optimism. It's kind of like, 'We're still standing.'"

Linda Cain, 52, awoke at 3:15 a.m. at her El Sobrante home and drove across the Bay Bridge to attend the event in honor of her late grandmother, Loretta O'Connor, who lived through the quake.

"Growing up she would talk about how this devastated her life," Cain said. "She loved San Francisco very much and she passed that on to me."

Communities up and down the San Andreas Fault, source of the magnitude-7.8 temblor, planned to commemorate the earthquake Tuesday.

In Santa Rosa, where 119 of the 7,500 citizens were killed, 119 volunteers dressed in vintage garb would walk by candlelight behind a horse-drawn hearse to the cemetery where 15 earthquake victims were buried in a mass grave.

San Jose, which was also hard-hit, has staged a geology exhibit called "It's Our Fault, Too." At the Exploratorium science museum in San Francisco, an artist sculpted a quivering San Francisco neighborhood in Jell-O.

The quake ranks as one of the costliest disasters in U.S. history, a benchmark to which later calamities are compared.

Historians generally agree that the city will crumble again in a future quake, but they disagree over whether people should leave it or love it.

Simon Winchester, the British author of "A Crack in the Edge of the World," a book about the disaster, told a forum Monday at the Commonwealth Club that he imagines a time hundreds of years hence when San Francisco is deserted.

"There will come a time when the city is knocked down again and again and again," he said.

Philip L. Fradkin, author of "The Great Earthquake and Firestorms of 1906: How San Francisco Nearly Destroyed Itself," told the group he has planted roots in the Bay Area and isn't budging.

"San Francisco fell, and it will fall again," said "And if we can't deal with the realities of history, we're lost."