Two powerful earthquakes hit Japan today, killing at least seven people, destroying hundreds of buildings and triggering a fire and radioactive leak at the world's largest nuclear power plant.

The first 6.8-magnitude quake stuck the northwest coast of Japan just after 10 am (0113 GMT) on a bank holiday. The epicentre was about 250 km (155 miles) northwest of Tokyo, with Kashiwazaki city in the Niigata prefecture worst affected.

The second “strong” earthquake, which was recorded at between 6.6 and 6.8 on the Richter scale, was centred off the west coast of Japan at 11.17pm (1417 GMT).

Most of the damage was reported after the first tremor. Flames and billows of black smoke poured from the Kashiwazaki nuclear plant, which automatically shut down during this morning’s quake.

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The fire, at an electrical transformer, was put out shortly after noon but engineers could not prevent the leak of 1.5 litres of radioactive water, which was flushed into the Sea of Japan.

The Tokyo Electric Power Co. said the water contained only a tiny amount of radioactive material. A company statement said that the leak had been isolated and that there had been no “significant change” in the seawater caused by the spillage, which contained a billionth of the proscribed limit for radioactive waste.

In Niigata and Nagano up to 800 people were injured and more than 500 homes were damaged, of those 300 were completely destroyed.

The tremors came as thousands of people travelled around the country during a national holiday, leaving many facing disrupted journeys home amid the transport chaos.

Shinzo Abe, the Prime Minister, cut short campaigning for an upcoming Upper House election and returned to Tokyo before heading to the affected northwest area. “We want to do all we can to ensure safety ... and to quell everyone’s concerns,” he said. “I want to get a picture of what happened and also want everyone to feel a little bit more secure.”

The Government has established an emergency office to deal with the quakes, while troops and extra emergency teams were being sent to help with rescue and clean-up efforts in Kashiwazaki city.

Some 2,000 people in Kashiwazaki were evacuated from their homes and about 37,000 households were without water.

The force of the first quake buckled seaside roads and bridges, and one metre-wide fissures could been seen in the ground along the coastline. Buildings swayed as far away as Tokyo.

“I was on the street, and there was strong sideways shaking. I couldn’t remain standing. One wall has collapsed,” Hiroki Takahashi, a petrol station worker, told the public broadcaster NHK. Two women in Kashiwazaki died, an official at the city's Central Hospital, said and NHK reported later reported that an additional two people had died.

Japan sits on top of four tectonic plates and is one of the world’s most earthquake-prone countries. Niigata was the site of an October 2004 earthquake with a matching magnitude of 6.8 that killed 65 people and injured more than 3,000.

The last major quake to hit the capital, Tokyo, killed some 142,000 people in 1923, and experts say the capital has a 90 per cent chance of suffering a major quake in the next 50 years.

In October 2004, a magnitude-6.8 earthquake hit Niigata, killing 40 people and damaging more than 6,000 homes. It was the deadliest to hit Japan since 1995, when a magnitude-7.2 quake killed 6,433 people in the western city of Kobe.

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