Japan’s network of advanced nuclear power plants are designed to shut down as soon as the earth shakes in one of the world’s most quake-prone countries, though a fire broke out in the turbine building of a nuclear plant in Onagawa.

More than eight million homes lost power, mobile and landline phone systems broke down for many and gas was cut to more than 300,000 homes, meaning many people could not heat their dark homes during a terrifying and cold night.

Japan’s military mobilised thousands of troops, 300 planes and 40 ships for the relief effort. An armada of 20 naval destroyers and other vessels headed for the devastated Pacific coast area of Honshu island, while air force jets flew reconnaissance missions over the disaster zone and Washington sent an aircraft carrier to provide help.

The wave set off tsunami alerts across the Pacific, as far away as South America, New Zealand and the US west coast.

California ordered hundreds of people to evacuate from coastal areas as tsunami waves swept in, while a half-metre surge was recorded on Mexico’s Pacific coast.

A Japanese ship with 100 people aboard was reportedly carried away, more than 300 houses were destroyed in the remote city of Ofunato and a dam broke in the northeast prefecture of Fukushima, with homes washed away.

“It was the biggest earthquake I have ever felt. I thought I would die,” said Sayaka Umezawa, a 22-year-old college studentwho was visiting the port of Hakodate, which was hit by a two-metre wave.

Prime Minister Naoto Kan said he had established an emergency headquarters for disaster response and called for calm from the public.

The quake, which hit at 2.46pm and lasted about two minutes, rattled buildings in greater Tokyo, the world’s largest urban area and home to some 30 million people.

It was felt in Beijing, some 2,500 kilometres away. A strong 6.6-magnitude quake which hit the west of Honshu island, away from the tsunami, early on Saturday was felt in Tokyo.

Millions who had earlier fled swaying buildings in the capital were left stranded in the evening after the earthquake shut down the city’s vast subway system.

The government urged people to stay near their workplaces rather than risk a long walk home, as highways leading out of the city centre were choked and hotels rapidly filled up.

There was major disruption to air travel and bullet train services. A passenger train with an unknown number of people aboard was unaccounted for on a line outside Sendai, Kyodo News reported.

The tsunami also submerged the runway at Sendai airport, while a process known as liquefaction, caused by the intense shaking of the tremor, turned parts of the ground to liquid.

Hours after the quake struck, TV images showed huge orange balls of flame rolling up into the night sky as fires raged around a petrochemical complex in Sendai.

A massive fire also engulfed an oil refinery near Tokyo as the quake brought huge disruption to Japan’s key industries. Tokyo share prices plummeted and the yen was down against the dollar.

The first quake struck just under 400 kilometres northeast of Tokyo, the US Geological Survey said. It was followed by more than 60 aftershocks, one as strong as 7.1.

Japan sits on the “Pacific Ring of Fire” and Tokyo is in one of its most dangerous areas, where three continental plates are slowly grinding against each other, building up enormous seismic pressure.

The government has warned of a 70 per cent chance that a magnitude-eight quake will strike within the next 30 years in the Kanto plains, home to Tokyo’s vast urban sprawl.The last time a “Big One” hit Tokyo was in 1923, when the Great KantoEarthquake claimed more than 140,000 lives, many of them in fires. In 1995 the Kobe earthquake killed more than 6,400 people.