San Francisco ? Sirens wailed and bells tolled through the city before dawn Tuesday as residents marked the moment 100 years earlier when the Great Quake shattered the city, killing thousands as it leveled buildings and touched off fires that burned for days.

A handful of centenarians who survived that devastation joined hundreds of other people for a moment of silence and a ceremony to remember one of the worst natural disasters in U.S. history.

The annual wreath-laying at Lotta’s Fountain, the downtown landmark where San Franciscans gathered after the quake to look for loved ones, was both a somber remembrance and a celebration of the city’s ability to rise from the ashes.

“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.”

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.

Chrissie Martenstein, 109, shares her memories of the 1906 earthquake with San Francisco Mayor Gavin Newsom. A dozen centenarians who survived the devastation 100 years ago were joined Tuesday by thousands of spectators for a ceremony to remember one of the worst natural disasters in U.S. history.

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 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.

In any case, it ranks as one of the costliest disasters in U.S. history, a benchmark to which later calamities are compared.

Newsom noted that the three years it took San Francisco to come back after 1906 bodes well for the hurricane-ravaged Gulf Coast.

“Don’t tell me you can’t rebuild,” he said. “We rebuilt, and we are stronger and better than ever.”

In this April 1906 photo, people on Sacramento Street watch smoke rise from fires that broke out following the earthquake in San Francisco.

Newsom asked the survivors, 10 women and two men, to share their memories of growing up in the scarred city. For most, they were spotty.

The oldest, Chrissie Martenstein, 109, said she remembered “a big shock and a great deal of misery,” while Violet Lyman, 102, could still recall seeing a cow running along a street and smelling smoke from the fires.

Government officials said they hoped to use the centennial to remind all Californians to prepare for another deadly quake by making plans to live for three days without power, water and other essentials.

A study released Monday determined that a repeat of that 1906 temblor today would cause 1,800 to 3,400 deaths, damage more than 90,000 buildings, displace as many as 250,000 households and result in $150 billion in damage.