The earthquake hit upper five on the Japanese scale, making it the most powerful to rattle the tremor-prone metropolis since February 1992.

Public broadcaster NHK said some 18 people were injured in the earthquake which sent objects flying off store shelves and books falling off cases.

The earthquake struck at around 4:35 pm (0735 GMT) with its focus located in the Boso peninsula of Chiba prefecture just outside the capital at a depth of 73 kilometers (45 miles) underground.

The meteorological agency revised up its Richter estimate from a preliminary magnitude of 5.7 and depth of 90 kilometers.

"We need to be cautious but there is little fear of aftershocks larger than the initial quake," a meteorological agency official said.

On the Japanese seismic scale which goes up to seven, an upper five intensity means people cannot walk around and television sets can tip over.

The Japanese scale measures how much places were shaken on the surface while the Richter scale measures the energy of the quake itself.

"The deeper the epicenter is, the lighter the tremor on the surface is. The epicenter of today's quake was deep, but the tremor was big," the meteorological official said.

Sirens were wailing in the streets of Tokyo and elevators came to a halt in tall buildings, a standard safety measure in the Japanese capital.

Five people sustained minor injuries as they went shopping in Saitama prefecture adjoining Tokyo, reports said.

Another woman cut her leg as drink bottles fell off the shelves at a shop, NHK said.

Among the other injuries, a woman in Chiba fell at a supermarket and two other people were hurt in Kanagawa prefecture south of Tokyo, the network said.

The government set up a taskforce at the prime minister's office to assess the situation.

A three-meter (10 foot) tall steel tower fell onto the roof of a house but nobody was injured, television footage showed.

The Asahi private network said the tremor caused at least two minor fires in Tokyo. Firefighters were seen climbing into an apartment building where a blaze seemed to have been extinguished.

Train platforms were swarmed by passengers, many of them on their way home from Tokyo Disneyland in Chiba.

Narita airport, the main international gateway to Japan, closed its runways to check any damage but re-opened them shortly afterwards.

Shinkansen bullet train services were also resumed although metropolitan subways and other train services remained halted.

The meteorological agency said there was no tsunami threat.

Japan is home to 20 percent of the world's major earthquakes and is always bracing for the dreaded "Big One" predicted to hit in the future.

The day's quake came hours after thousands of volunteers and rescue workers took part in one of Japan's largest-ever tsunami drills, anxious to prevent a repeat of the devastation seen in last year's Indian Ocean catastrophe.

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