By last night the death toll was estimated at more than 1,000, most of them drowned — a toll that looks sure to rise significantly in the coming days.
And fears of a potential nuclear disaster were raised by the news that a power station was forced to release radioactive vapour to ease pressure on the reactor.
The massive 8.9 magnitude quake hit the north of the country 230 miles (373km) from Tokyo. Near the epicentre and in the worst-hit Miyagi Prefecture, houses toppled over or collapsed, burying dozens of people. Extraordinary television images showed a tide of muddy water sweeping cars and houses across open land at high speed.
Last night there were reports that a second earthquake had struck Japan. A 6.6 earthquake struck the central, mountainous part of the country — far from the original quake's epicentre.
The day was punctuated with shocking images of devastation and public paralysis. Police reported that a ship carrying more than 100 people was swept away in the giant tsunami that crashed into the country's north-east.
A major blast rocked a petrochemical complex in Chiba, outside the capital. The earthquake shut Japan's busiest main international airport, brought the capital's entire train network to a halt and sent thousands of office workers spilling out on to the streets.
There were also fears over the safety of the country's nuclear power stations. In Miyagi, fire broke out at the Onagawa nuclear plant and at least three other plants were automatically shut down.
The government declared a state of emergency, Japan's first, at the Fukushima No 1 plant after reporting that its cooling system had failed.
A report by Kyodo News said that one reactor in the plant “could not be cooled” and that residents had been told to evacuate the area.
Water also reportedly spilled from pools containing fuel rods at the world's largest nuclear power plant, Kashiwazaki-Kariwa plant in Niigata Prefecture, but there were no reports of radioactive leaks.
At train stations in the centre of Tokyo thousands of commuters waiting to board trains gasped and clung on to each other or hunkered down as the earthquake struck, rocking platforms and buildings and sending glass showering down from the roof.
In the city's business districts office workers in safety helmets crowded the streets, nervously glancing upwards for falling debris and glass.
“It just seemed to go on for ever,” said Nahoko Ishii, who was waiting for a train in Tokyo Station when the earthquake struck. “I've never experienced anything that terrifying in my life.”
Mobile phone networks crashed as millions tried to call family and friends in the minutes and hours after the quake.
The yen fell in currency markets immediately afterwards. As the country absorbed the devastating news, Prime Minister Kan appeared on television to commiserate with grieving families and to ask the public to stay vigilant and to keep abreast of news reports.
“I ask everyone to act calmly,” he said.
Japan's Meteorological Agency said the quake was the most powerful in the country's long history of recorded seismic activity, exceeding even the 1923 disaster that levelled much of Tokyo and nearby Yokohama and killed more than 100,000 people.
Only the country's state-of-the-art building and warning systems prevented the death toll from being much higher.
News of deaths and injuries began flooding in yesterday in the hours after the quake struck.
There were early reports of deaths in chemical spills and building collapses. Later the toll soared as 200 to 300 bodies were found in one ward of Sendai alone. As authorities account for the missing and take a more detailed approach, the toll in the days ahead could rise considerably.