Japanese TV showed cars, ships and buildings swept away by a vast wall of water after the 8.9-magnitude quake.
A state of emergency has been declared at a nuclear power plant but officials said there were no radiation leaks.
At least 60 people have been killed by the quake, which struck about 400km (250 miles) north-east of Tokyo.
The region has been rocked by a number of powerful aftershocks.
The tremor, measured at 8.9 by the US Geological Survey, hit at 1446 local time (0546 GMT) at a depth of about 24km.
Coastal areas in the Philippines, Hawaii and other Pacific islands were evacuated ahead of the tsunami's expected arrival.
New Zealand later downgraded its alert to a marine threat, meaning strong and unusual currents were expected.
Wall of water Strong waves hit Japan's Miyagi and Fukushima prefectures, officials said, damaging dozens of coastal communities.
Kyodo news agency said a 10-metre wave (33ft) struck the port of Sendai in Miyagi.
Japan's NHK television showed a massive surge of debris-filled water sweeping away buildings, cars and ships and reaching far inland.
Motorists could be seen trying to speed away from the wall of water.
Farmland around Sendai was submerged and the waves pushed cars across the runway of the city's airport. Fires broke out in the city's centre.
The UN's nuclear agency said four nuclear power plants had shut down safely.
Police and local officials said at least 40 people had been killed in the earthquake and tsunami. It is believed the death toll could rise significantly.
A ship carrying 100 people was swept away, Japanese media reported, quoting police in Miyagi. It is not clear what happened to the ship and its passengers.
In Iwate prefecture, also near the epicentre, an official said it was difficult to gauge the extent of the destruction.
"Roads were badly damaged and cut off as the tsunami washed away debris, cars and many other things," said Hiroshi Sato, a disaster management official in Iwate.
The earthquake also triggered a massive blaze at an oil refinery in Ichihara city in Chiba prefecture near Tokyo, engulfing storage tanks.
There were reports of about 20 people injured in Tokyo after the roof of a hall collapsed on to a graduation ceremony.
Residents and workers in Tokyo rushed out of apartment buildings and office blocks and gathered in parks and open spaces as aftershocks continued to hit.
A state of emergency has been declared at a nuclear power plant but officials said there were no radiation leaks.
At least 60 people have been killed by the quake, which struck about 400km (250 miles) north-east of Tokyo.
The region has been rocked by a number of powerful aftershocks.
The tremor, measured at 8.9 by the US Geological Survey, hit at 1446 local time (0546 GMT) at a depth of about 24km.
A tsunami warning was extended across the Pacific to North and South America.
The Red Cross in Geneva warned that the tsunami waves could be higher than some Pacific islands, Reuters news agency said. Coastal areas in the Philippines, Hawaii and other Pacific islands were evacuated ahead of the tsunami's expected arrival.
New Zealand later downgraded its alert to a marine threat, meaning strong and unusual currents were expected.
Wall of water Strong waves hit Japan's Miyagi and Fukushima prefectures, officials said, damaging dozens of coastal communities.
Kyodo news agency said a 10-metre wave (33ft) struck the port of Sendai in Miyagi.
Japan's NHK television showed a massive surge of debris-filled water sweeping away buildings, cars and ships and reaching far inland.
Motorists could be seen trying to speed away from the wall of water.
Farmland around Sendai was submerged and the waves pushed cars across the runway of the city's airport. Fires broke out in the city's centre.
A state of emergency was declared at the Fukushima power plant after the cooling system failed in one of its reactors when it shut down automatically because of the earthquake.
The local government urged 2,000 nearby residents to evacuate the area as a precaution. Prime Minister Naoto Kan said no radiation leaks at that power plant or any of the other reactors in the quake-hit zone had been detected.The UN's nuclear agency said four nuclear power plants had shut down safely.
Police and local officials said at least 40 people had been killed in the earthquake and tsunami. It is believed the death toll could rise significantly.
A ship carrying 100 people was swept away, Japanese media reported, quoting police in Miyagi. It is not clear what happened to the ship and its passengers.
In Iwate prefecture, also near the epicentre, an official said it was difficult to gauge the extent of the destruction.
"Roads were badly damaged and cut off as the tsunami washed away debris, cars and many other things," said Hiroshi Sato, a disaster management official in Iwate.
The earthquake also triggered a massive blaze at an oil refinery in Ichihara city in Chiba prefecture near Tokyo, engulfing storage tanks.
There were reports of about 20 people injured in Tokyo after the roof of a hall collapsed on to a graduation ceremony.
Residents and workers in Tokyo rushed out of apartment buildings and office blocks and gathered in parks and open spaces as aftershocks continued to hit.
