Belly-up ships, twisted cars and debris from shattered buildings today crashed through the streets of Japanese port towns that were turned into black rivers by a monster tsunami.

A muddy river filled with rubble -- some of it on fire and belching smoke -- raced across rice fields and through towns, aerial television footage showed in one of the worst-hit areas, Miyagi prefecture.

A schoolboy was swept away there by the deadly waters and there were grave fears the toll would keep climbing sharply from the more than two dozen reported dead as a cold night settled over Japan.

The huge wall of sea water unleashed by Japan's worst quake on record hit the Pacific coast of Honshu island, sweeping away whole houses and turning harbour areas into scenes of utter devastation.

The masses of water overwhelmed coastal defences and swallowed up many square kilometres of land in the region in scenes reminiscent of the devastation triggered by the 2004 Indian Ocean tsunami.

"I've never seen anything like this," said Ken Hoshi, a local government official in Ishinomaki, a port city in Miyagi prefecture.

"The water came as far as to the train station," hundreds of metres away from the coast, the 41-year-old official said as his city turned into a flood zone.

"I'm worried because I can't contact my family. But because it's my duty, I'm braced to spend the night here," he said from his fourth-floor office.

In nearby farming areas, the flood swept away houses like toys and devoured dusty fields still barren in early spring, before the deadly tide was stopped inland by the embankments of raised highways.

In far-northern Aomori, at least five ocean-going ships, some flipped belly-up to expose their red hulls, raced inland over shattered sea defences, rows of trees and harbourside shopping streets.

In Ibaraki large houses were seen floating through the town, and elsewhere dozens of cars bobbed in the waters like corks.

The quake that triggered the tsunami tore cracks into roads, pushed manhole covers and their attached pipes out of the ground like small towers and dumped the contents of supermarket shelves on the ground.

Completing the apocalyptic scenes, elsewhere in Japan scores of fires broke out as gas mains were severed. An oil refinery was ablaze in Chiba prefecture outside Tokyo, belching acrid smoke into the skies.

As aftershocks kept rocking the country, the army deployed troops to help those in need, a rescue effort that became more complex as darkness fell.