The tsunami was triggered by Japan’s biggest earthquake since records began.
The government has declared a state of emergency at five nuclear reactors as cooling systems failed.
The an 8.9-magnitude tremor struck in the afternoon local time on Friday off the coast of Honshu island at a depth of about 24km, 400km (250 miles) north-east of Tokyo.
It was nearly 8,000 times stronger than last month’s quake in New Zealand that devastated the city of Christchurch, scientists said.
Japanese police told Kyodo news agency 398 people are confirmed to have died and 805 more are missing; other media outlets put the death toll above 400.
The country’s military has mobilised thousands of troops, 300 planes and 40 ships for the relief effort.
Prime Minister Naoto Kan visited the disaster zone by helicopter early on Saturday.
Among the places he visited was the Fukushima nuclear plant, damaged during the quake.
Mr Kan confirmed that a small amount of radioactive material had been released into the air after technicians were forced to release gases from the reactors in a bid to lower pressure.
Officials have insisted that there is no risk to people in the area, but have evacuated thousands as a precaution.
Meanwhile, rescue teams from South Korea, Australia, New Zealand and Singapore are due to arrive later. US President Barack Obama said a US aircraft carrier was already in Japan and another was on the way.
The quake triggered a tsunami up to 10m (30ft), with waves of 7m battering the Japanese coast.
A muddy torrent of water swept cars and homes far inland, turning residential areas and paddy fields into a lagoon of debris-filled sea water.
One of the worst-hit areas was the port city of Sendai, in Miyagi prefecture, where police said between 200 and 300 bodies were found in one ward alone.
The town of Rikuzentakada, in Iwate prefecture, seemed mostly under water, with barely a trace of any buildings.
Japan Railways said it could not trace four trains along the north-eastern coast, and a ship carrying 100 people was also reported missing.
Several fires were reported in Kesennuma, in Miyagi prefecture, and one-third of the city was also said to be under water.
Some 1,800 homes were reported to have been destroyed in the city of Minamisoma, Fukushima prefecture.
And a dam burst in north-eastern Fukushima prefecture, sweeping away homes, Kyodo reported.
More than 50 aftershocks – many of them more than magnitude 6.0 – have rattled the country.
“It was the biggest earthquake I have ever felt. I thought I would die,” said Sayaka Umezawa, a 22-year-old student who was visiting the port of Hakodate.
In central Tokyo, a number of office workers spent the night in their offices because the lifts stopped working.
Millions of commuters were stranded overnight and others walked home after train services were suspended.
At least 20 people were injured in Tokyo when the roof of a hall collapsed on to a graduation ceremony.