Japan's national police say 151 people are confirmed dead so far, with another 531 missing.
Those figures are expected to rise much higher. Police in Miyagi prefecture said an additional 200 to 300 bodies had been found washed up on shore, while reports are also coming in of a ship swept away with more than 100 people on board and four passenger trains missing.
The defence ministry estimates the final death toll at over 1,000. Most deaths are thought to be caused by drowning.
As well as the loss of life, the disaster has caused massive damage. More than 5,000 homes have been destroyed.
The government declared an atomic power emergency after the cooling system at its Fukushima nuclear plant failed.
Radiation levels 1,000 times higher than usual were measured in the plant's control room, leading to fears of a leak. Radiation was eight times above the normal level outside the plant. Japan's nuclear safety authority says there is no immediate health hazard, but anyone within a 10-kilometre radius of the facility has been ordered to leave the area.
Meanwhile a fire at the Onagawa nuclear plant has been safely extinguished, according to its operator.
Millions of households in north-east Japan remain without electricity, Tohuko Electric Power company reported.
In the capital, millions of people were left stranded when rail services were suspended for the entire greater Tokyo area. Schools and stadiums have been turned into temporary shelters, while restaurants and food shops have been ordered to provide people with toilets and drinking water.
Other countries have been quick to offer assistance. The United Nations will do "anything and everything" to help, said UN Secretary-General Ban Ki-moon. More than 68 teams from more than 45 countries are on standby to join the relief effort, according to the UN's Office for the Coordination of Humanitarian Affairs (OCHA).
The World Bank has also offered to spend specialists to support the recovery effort, even though Japan - one of the world's richest countries - is not usually eligible for its aid.
The Red Cross has launched a special website to help people contact friends and family in Japan; it can be found here.
The impact elsewhere
The earthquake triggered huge waves across the Pacific:
- In the US, tidal waves 1.8 metres high hit Hawaii and surges 2.4 meters high reached the coasts of California and Oregon. Boats were crushed and one person swept out to sea. Evacuations have been ordered in several counties.
- Waves up to 70 centimetres high hit Mexico's coastal areas of Baja California.
- Chile has ordered an evacuation of coastal zones at risk from the tsunami. Waves are heading for Easter Island.
- Ecuador's state oil company has suspended crude oil exports due to the tsunami risk.
- Nicaragua says some 100,000 people along the coastline are potentially
- New Zealand upgraded its tsunami warning early on Saturday. Waves of more than one metre are expected.