Japanese radicals reportedly sabotaged train lines in six places and exploded a bomb outside the Japanese government's Defense Agency headquarters early this morning, incidents certain to raise new fears over security at the economic summit that President Reagan and other world leaders will attend here next week.

The attacks, reported by Japanese television, were the latest in a series of incidents that police have been unable to prevent despite unprecedented security in the Tokyo area and a nationwide hunt for the radicals.

The attacks, which caused only minor damage and disruption, seemed to have been timed to protest government ceremonies scheduled for later today marking Emperor Hirohito's 60th year on the throne. Radicals had said they would disrupt that event as well as the summit.

Two of the rail attacks reportedly hit the Tokyo-Osaka "bullet train" line, Japan's major rail artery. Fires broke out in two places in the line's electric and communications cables. Traffic had been restored in the Tokyo-Osaka direction several hours later, but the other direction was still halted.

Earlier, fires or automatic incendiary devices were found in four places on various commuter rail lines in Osaka, according to TV reports. Service was disrupted for only about 10 minutes and ticket-selling computers in eight stations were affected.

Late last year, radicals attacked Tokyo commuter rail lines in more than 30 places simultaneously and paralyzed the entire city for a morning. Millions of commuters were stranded.

Today's explosion occurred shortly after midnight in the public restroom of a park adjoining the main compound of the Defense Agency. It injured one man, who was taken to a hospital by authorities.

Police said they believe the man was transporting or planting the bomb when it went off accidentally.