A magnitude 8.5 earthquake struck off the east coast of Japan on Saturday, shaking buildings in Tokyo, but there was no danger of a tsunami and no immediate reports of damage since the quake was extremely deep.

There were no reports of further irregularities at the Fukushima Dai-ichi nuclear plant, damaged four years ago in a massive 9.0 magnitude quake, or any other nuclear facilities.

The Shinkansen high speed train line had briefly stopped between Tokyo and Osaka due a power outage.

The quake, centred off the Ogasawara islands south of Tokyo, occurred at a depth of 590 km and, unusually, was felt throughout much of Japan, but there were no immediate reports of significant damage or injuries, though the Tokyo Fire Department had received calls of people suffering injuries through falls, NHK national television said.

In Chiba, just east of Tokyo, loudspeaker broadcasts called on people to remain calm, but there were no reports of damage.

"Since it was magnitude 8.5 this was a very big quake, but fortunately it was very deep at 590 km," Naoki Hirata, an earthquake expert at the University of Tokyo's Earthquake Research Centre, told NHK.

"But the shaking was felt over a broad area... Fortunately, because it was deep, there is little danger of a tsunami."

Tokyo Electric Power Co said there were no abnormalities at the crippled Fukushima nuclear power plant following the quake.

On March 11, 2011, a massive 9.0 magnitude quake touched off a tsunami that left nearly 20,000 people dead and nuclear meltdowns at the Fukushima Dai-Ichi nuclear plant.

(Reporting by Edmund Klamann, Kiyoshi Takenaka, Linda Sieg, Mari Saito and Joseph Radford, writing by Elaine Lies; Editing by Pravin Char and Rosalind Russell)