Cars, ships and buildings were swept away by a wall of water after the 8.9-magnitude tremor, which struck about 400km (250 miles) north-east of Tokyo.
A state of emergency has been declared at a nuclear power plant, where pressure has exceeded normal levels.
Officials say 350 people are dead and about 500 missing, but it is feared the final death toll will be much higher.
In one ward alone in Sendai, a port city in Miyagi prefecture, 200 to 300 bodies were found.
