TOKYO - A strong earthquake jolted northern Japan early on Thursday, injuring dozens of people, trapping hundreds in halted trains and cutting off electric power to thousands of homes.

The Japan Meteorological Agency said there was no threat of a tsunami from the quake, which struck at 12:26 am JST (1526 GMT Wednesday) and had a preliminary magnitude of 6.8 and could be felt as far away as Tokyo.

Chief Cabinet Secretary Nobutaka Machimura told reporters that at least 26 people had been injured and 6,000 homes were without electricity. A National Police Agency official said the 26 had suffered minor injuries such as cuts from broken glass.

"Once it's light, we'll have a better grasp of the situation and helicopters will be able see things like landslides," Machimura said after the government set up an emergency task force at the prime minister's official residence.

National broadcaster NHK put the injured toll at 56.

The focus of the quake was 120 km (75 miles) below the surface of the earth in Iwate prefecture, a mountainous, sparsely populated region, the agency said.

"I woke up immediately. It felt like it was shaking for a long time. Books and other things that were piled up fell on the floor. All the doors were open and things were shattered," Sho Koseki, a city official in Hachinohe, about 550 km northeast of Tokyo, told Reuters by telephone.

Koseki said that military troops had arrived in the area, and the Defense Agency said that military planes were flying over the area to try to assess the extent of the damage.

STUCK IN TRAINS

East Japan Railway Co said more than 600 people were stuck on five trains that were halted due to the quake.

There was a fire in one building in Hachinohe, a city with a population of about 240,000, and national broadcaster NHK showed fire engines driving through the streets towards the scene of the blaze, but the fire was soon extinguished and there were no reports of injuries.

Some parts of highways were closed to traffic in the region, a mountainous and sparsely populated part of Japan, but they were later reopened, NHK reported.

TV footage showed offices with papers strewn around on the floor and some with parts of ceilings fallen down.

"People in the store were trying to protect dishes from breaking," a young woman in the northern city of Sendai told NHK.

"I thought the big one had finally come."

The Japan Meteorological Agency warned of landslides in some areas, where strong rain was expected in coming hours.

Tohoku Electric said its nuclear facilities in the area were operating normally after the quake, except for one unit that was already off-line for maintenance work.

Tokyo Electric said its nuclear plants further south had not been affected.

Nippon Oil said its 145,000 barrels per day Sendai refinery was operating normally after quake.

But Tohoku Electric said it had manually shut down a 250-megawatt oil-fired power plant in Aomori and Nippon Steel halted steel output and shut down a 149-megawatt coal-fired power plant at its Kamaishi works for inspection.

Earthquakes are common in Japan, one of the world's most seismically active areas. The country accounts for about 20 percent of the world's earthquakes of magnitude 6 or greater.

Thursday's quake follows a string of earthquakes in the same region, the first of which in mid-June killed at least 10 people and left as many again missing.

In October 2004, an earthquake with a magnitude of 6.8 struck the Niigata region in northern Japan, killing 65 people and injuring more than 3,000.

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