(Reuters) - When the massive earthquake struck northern Japan on March 11 and triggered a tsunami alarm in her Pacific Coast fishing port, civil servant Takako Suzuki knew exactly where to shelter.

At least, she thought she did.

Suzuki, 40, followed instructions broadcast on loud speakers and headed up to the third floor of the civic center that her city of Rikuzentakata had designated as a tsunami evacuation point.

As she ran up the steps, a 14-meter (46 feet) tsunami smashed through the building, its swirling black water swallowing people around her.

"Black water broke into the building through the windows with a rumbling noise, and I was just helplessly tossed around by the water. I don't even remember how I was swept along."

She found herself in a cluster of a dozen people, dragged into a room where the water kept rising to about 10 cm (5 inches) from the ceiling, Suzuki told Reuters journalists she had accompanied back to the scene of her life-and death struggle just over three weeks earlier.

"I was thinking 'if the water rises a little bit, I'm finished' - but fortunately the water suddenly stopped rising and began receding."

The 12 survivors, still half-submerged as night fell and cold winds blew in through missing windows, huddled together. Around them were bodies of those who didn't make it, she said.

"The building lurched because it was still filled with water and aftershocks kept shaking the building, causing debris to fall down on us. That was scary," said Suzuki.

"We were wondering if morning would ever come, if help would ever come," she said.

Their ordeal ended almost 24 hours later when Suzuki and 10 others were plucked from the building by a rescue helicopter operated by the Self Defense Force, Japan's military. One of them, an elderly man, died during the night. Suzuki estimated that her group of 11 survivors were among 60 people who fled to the civic center. The Japanese magazine AERA reported 100 people sought refuge there.

"It's a mystery to me how I am alive," she said.

"I thought I was definitely going to die at the moment, but I'm a mother of three, so I was holding up through it only by thinking that I wanted to see my children again," Suzuki said, her calm voice suddenly cracking.

Rikuzentakata, one of the most devastated towns along more than 250 km (150 miles) of shattered Japanese coastline, counts 1,066 dead and 2,041 missing from a population of 23,000.

Suzuki's family home was on higher ground and untouched by the tsunami that flattened her workplace and virtually every structure near sea level in the small Iwate Prefecture city.

She has joined her colleagues and volunteers helping 160 refugees at a shelter set in a Buddhist temple near the devastated center of the city. "My life was given back to me," she said and vowed to pour her energy into helping others get back on their feet after Japan's worst humanitarian disaster since World War Two.

(Writing by Paul Eckert, editing by Jonathan Thaatcher)