A powerful earthquake has struck off the Japanese coast, shaking buildings in Tokyo and setting off car alarms.

The US Geological Survey reported the earthquake had a magnitude of 7.8, while the Pacific Tsunami Warning Centre said it had a magnitude of 8.5.

Despite the huge power of the quake on Saturday, there was no risk of a tsunami, the Pacific Tsunami Warning Centre reported.

Buildings swayed for about a minute as the quake built in intensity about 8.30pm (21.30 AEST).