The National Police Agency said four women and three men, all in their 70s or 80s, died from injuries sustained in the earthquake.

At least 692 people were hurt and taken to hospitals in Niigata and Nagano prefectures, local authorities and hospital officials said.

More than 300 buildings were completely destroyed and another 212 were partially damaged, according to the Fire and Disaster Management Agency.

The government set up a crisis management centre while Prime Minister Shinzo Abe broke off from election campaigning to race to the scene of the worst damage in Niigata prefecture.

Rescue workers were hunting for anyone buried alive in the wreckage after nearly 300 buildings were flattened by the quake, which shook skyscrapers in Tokyo more than 200km from the epicentre.

"When the earthquake hit, I was out on my boat and I felt this swing," said Susumu Ishiguro, the owner of a fishing shop in the worst-hit city of Kashiwazaki on the Sea of Japan (East Sea).

"I came back to the port and I found my house was a complete mess. I think all the old houses got crushed," he said.

The four women and one man confirmed dead were all in their 70s or 80s, police said. At least 644 people were hurt and taken to hospitals, local officials said.

Niigata was hit by another 6.8 Richter-scale earthquake in 2004 that killed 67 people, most of them elderly who died in the days and weeks after the first tremor from stress and fatigue.

The latest quake triggered 50-centimetre (1.5-foot) tsunami waves and was followed by some 65 aftershocks that could be felt by people, the meteorological agency said.

Raising fears among residents, smoke billowed for hours from a blaze at a nuclear power plant, one of the largest in the world, which supplies electricity to the Tokyo region.

Plant officials said the reactors shut down automatically and that there was no radiation danger.

More than 300 buildings were completely destroyed and another 212 were partially damaged, according to the Fire and Disaster Management Agency.

Abe, dressed in a relief worker's uniform, headed to the scene by military helicopter, cutting short a campaign stop ahead of July 29 elections.

Abe, who is struggling in opinion polls, said he gave instructions to his government that "all possible measures be taken to ensure the safety of residents, secure lifelines to them and relieve their anxieties."

The quake also triggered mudslides in Kashiwazaki, where soil was already loose after a major typhoon at the weekend, which left four people dead or missing and flooded hundreds of homes across Japan.

Monday was a bank holiday in Japan, so financial markets and many offices were closed.

Service on Japan's famed bullet trains was temporarily suspended as a precaution after the earthquake, which severed power to some 21,000 households.

Japan lies at the junction of four tectonic plates and is hit by about 20 percent of the world's most powerful earthquakes.

In January 1995, a 7.3-magnitude quake destroyed much of the western metropolis of Kobe, killing more than 6,400 people.