TOKYO -- Police said Tuesday they suspect an explosion near a U.S. army base southwest of Tokyo was an attack by Japanese extremists and mobilized some 140 police officers to investigate.

The blast occurred shortly after 11:00 P.M. Monday (1400 GMT) at a public park 500 meters (yards) from the U.S. army's camp Zama, in Kanagawa, 40 kilometers (25 miles) southwest of Tokyo, police said.

No one was injured in the attack, a Zama police spokesman told AFP.

Police found a projectile launcher, a metal pipe about 55 centimeters (22 inches) long and about 5.5 centimetres in diameter, at the park as well as scorch marks on the ground, he said.

"The metal launcher was aimed at the U.S. base and we suspect it was a guerrilla attack," the official said, referring to Japanese radicals.

Police found a second launcher, identical in design to the first pipe, and batteries in a thicket outside the park.

"We have yet to find any explosives and I cannot confirm any projectiles actually entered the camp," he said.

Police have yet to detain any suspects in relation to Monday's attack and no one has so far claimed responsibility, the official said, contradicting comments by U.S. military officials that a couple of people were already in the custody of Japanese authorities in connection with the explosion.

The spokesman said at least two projectiles were fired as neighbors reported hearing several explosions.

Nearby residents had told police they had seen two men in their 30s running away from the park.

Camp Zama is a major U.S. army base, housing 2,000 military personnel assigned to numerous units including the 1st battalion, special forces group, signals, military intelligence and logistics units.

A Japanese leftist radical group, the Kakurokyo (revolutionary workers) faction, claimed responsibility for a July 1993 homemade rocket attack at a U.S. air force base in Yokota, western Tokyo.

No one was hurt in the attack, which the group said was in opposition to the Group of Seven summit being held in Tokyo, according to contemporary press reports.

A week later four steel rockets were fired by Japanese leftist radicals at the Zama Camp and fell inside the grounds. No one was hurt in that attack either.

During a previous Japanese-hosted group of seven summit in 1986, the Yokota base was also the target of an extremist attack with a crude projectile by the radical wing of the Chukaku-ha (middle core faction).

In February this year, there was an explosion at a high school baseball field in Yokohama, about 30 kilometers south of Tokyo. The baseball ground was about 400 meters away from the U.S. navy's Koshiba fuel terminal. Police found a steel ball near the terminal. No one was injured.