More than 800 people were injured by the quake in Honshu island's Niigata prefecture, and buildings swayed as far away as Tokyo.

Thousands were evacuated from their homes instead of enjoying a national holiday.

A statement from Tokyo Electric Power Company said 1.5 liters of water containing radioactive materials had leaked from the No6 unit at its Kashiwazaki-Kariwa nuclear power plant.

The contaminated water had been released into the ocean and had no effect on the environment, the company said. Tepco executives said earlier there were no radiation leaks at the plant, where reactors automatically shut down for checks.

The fire in an electrical transformer at Kashiwazaki-Kariwa was quickly extinguished, said company spokesman Yoshinobu Kamijima. But it was unclear when Tepco could restart three power units, he added.

Among the dead, two women in their 80s were killed when their houses collapsed during the magnitude 6.8 tremor. A police spokesman confirmed the deaths of seven elderly people, and a 77-year- old man was reported missing after going for a walk before the quake hit at 10.13am.

An 84-year-old woman was rescued from the wreckage of her collapsed house some five hours after the quake.

By then, about 7,800 people had fled their homes to nearly 100 evacuation centers as scores of aftershocks rattled the area.

They included another 6.8 magnitude tremor, although this was under the sea off the west coast.

Troops and extra emergency teams were being sent to help with rescue and relief efforts, while Prime Minister Shinzo Abe cut short campaigning for parliamentary elections to inspect the area.

Authorities set up an emergency office to deal with the quake, which officials said had damaged about 500 buildings.

Teacher Harumi Mikami, 55, was at her school in Kashiwazaki City, near the epicenter in Niigata prefecture about 250 kilometers northwest of Tokyo.

"First there was a sharp vertical jolt and then it shook sideways for a long time and I couldn't stand up," she said. "Tall shelves fell over and things flew around."

The quake halted gas service to about 35,000 homes and disrupted the water supply to all of Kashiwazaki, a city with a population of around 95,000 that was hardest hit by the quake.

About 25,000 homes in Niigata prefecture were without electricity.

Houses, many of them wooden with traditional heavy tile roofs, were flattened, a temple roof caved in and roads cracked.

Bullet trains stopped services in northern Japan after the quake but resumed about 11 hours after the quake. A local train toppled from the rails, but media said no one was injured.

Landslides closed several local roads, and rain was forecast in the area for two days, raising worries about more slippage.

Japan is one of the world's most earthquake- prone countries, with a tremor occurring at least every five minutes.

Niigata was hit in October 2004 by a quake with a matching magnitude of 6.8 that killed 65 people and injured more than 3,000.

That was the deadliest quake in Japan since a magnitude 7.3 tremor hit Kobe city in 1995, killing more than 6,400 people.

REUTERS