A strong earthquake registering 5.7 on the Richter scale shook Tokyo and its vicinity, lightly injuring five people, swaying buildings in the heart of the capital and disrupting transport.

The earthquake struck at 4:35 pm (0735 GMT) with its focus located in the Boso peninsula of Chiba prefecture just outside of the capital at a depth of 90 kilometres (45 miles) underground.

Five people sustained minor injuries at a shopping center in Saitama prefecture adjoining Tokyo, Kyodo News said. The government set up a taskforce at the prime minister's office to assess the situation.

Sirens were wailing in the streets of Tokyo and elevators came to a halt in tall buildings, a standard safety function in the earthquake-prone metropolis. Books fell off the shelves in houses.

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 fire 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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