LOS ANGELES, Sept. 5 – The Los Angeles County Hall of Justice, site of the Sharon Tate murder trial, was rocked by a bomb explosion on its sixth floor today shortly after midnight. The blast blew out a wall and broke a six-inch water main, but caused no injuries, sheriff’s deputies said.

The bomb exploded in a restroom adjacent to the district attorney’s office, deputies said.

Water from the broken main caused an ankle-deep flood on floors one through six, but caused no damage to official court records housed in the 14-story building, Dist. Atty. Evelle J. Younger told a news conference.

Younger said a man called the county offices two minutes after the blast to warn that an explosion would occur within three minutes. Younger said he did not know whether whoever planted the bomb was mistaken over its timing or whether the call was a coincidental prank.

The blast was “barely felt” in the upper portions of the building, including the 10th through 14th floor jail complex that houses some 2,800 prisoners including Charles M. Manson, deputies said.

Manson, leader of a hippie type clan, and three women codefendants are being tried in an eighth-floor courtroom on charges of murder-conspiracy in the deaths of actress Sharon Tate and six others in August 1969.

One deputy, whose office is on the second floor, said, “I felt a slight tremor and heard a big boom.” When the bomb went off at 12:55 a.m., the deputy said, “We all ran out of our of free with our pistols drawn.”

He said the Hall of Justice’s doors are open during the predawn hours and the bomber or bombers may have slipped past security guards posted throughout the building.

Construction of the explosive device was not immediately determined, deputies said. Its blast blew out a 9-by-12 foot concrete wall and uprooted plumbing in the restroom, they said.

“The bomb peeled the ceiling back like a tin can” in the restroom, a deputy said. “Water from the main came down the stairways like a flash flood.”

By JOHN BREWER