Anti-nuclear demonstrators delayed construction work at the Diablo Canyon nuclear power plant for several hours today, the first time demonstrators had managed to disrupt work at the plant they have been trying to blockade for two days.

The protesters, who have focused on the much-delayed Diablo Canyon project as a symbol of the nuclear industry, took advantage of a communications breakdown at the plant and the nerve of one demonstrator to block the main gate for more than four hours.

The demonstrator, identified as Itara Katherine O'Connell, lay down in front of the first of about 15 buses carrying construction workers into the plant. She did not move even when the driver rolled so close that one of his front tires almost touched her.

Several other demonstrators joined her in front of the bus and when, after 20 minutes, no deputies appeared to arrest the blockaders, the buses turned around and left. San Luis Obispo County Undersheriff Arnie Goble said later that his deputies assigned inside the plant were not told that the buses would try to enter.

Early this afternoon the deputies appeared dressed in military fatigues and arrested or pushed out of the way several demonstrators, then opened the gate and let the buses through.

Sgt. Leon Cole of the sheriff's department said 117 people were arrested today. Most of them were picked up in the main gate area, but some were on back trails and on the beach by which demonstrators have tried to reach the the plant, which is on the California coast 150 miles northwest of Los Angeles.

A total of 682 people have been arrested in the two days of protests, and nearly all remain in custody, the men in a local junior college gym, and the women in an unused part of a minimum-security men's prison, while officials struggle to complete paperwork for their arraignments.

Six women were released today on their own recognizance after they promised not to return to the demonstration, and one woman was released for treatment at a hospital after it was discovered that she had broken her arm while on a hike to the plant gates. Demonstrators said two men were also released, one because he had broken his leg falling off a cliff on the coast, and one because he was suffering epileptic seizures.

Most of the other protesters are expected to remain in custody for some days, Cole said, because the local municipal court can arraign a maximum of 200 a day, beginning Thursday. Demonstration organizers have said many people will refuse a chance to be released on bail if everyone in the demonstration is not allowed freedom on his own recognizance.

Demonstrators standing outside the main gate of the plant today seemed buoyed by their brief success at disruption, after the mass arrests of the first day made it possible for the plant to carry on work without any delays. But their dwindling numbers appeared to signal the end of the protest, which now appears to have only symbolic value in warning Americans of the protesters' perception of the potential hazards of nuclear fuel.

The protest began Tuesday after 1,500 to 2,000 people had gathered over several days on a temporary farmland camping site 12 miles east of the plant. Many of those have identified themselves only as supporters of the protest who do not plan to risk arrest.

Asked if only small numbers of demonstrators would now make the daily attempt to block the plant's gate, Kristin Leimkuhler, speaking for the anti-nuclear Abalone Alliance, said, "Well, we don't have that many people left."

Robert Thornberry, manager of the nuclear plant, told reporters after the buses had failed to enter this morning that the delay was "a slight disruption." He said preparations for the first loading of nuclear fuel, which the protesters have vowed to try to stop, were not affected because workers making those preparations are living at the plant site.

He said the more than 500 construction workers delayed today were working on the second of the plant's two units, which is several months behind the completed first unit. Together the two reactors could provide 20 percent of northern California's electricity.

"Their rights are interfering with my rights," said Mark Fletcher, 26, a welder for Pullman Power Products and one of the workers kept off the plant this morning.

He and several other workers, interviewed on the streets of this little beach town as they waited for the gate to be cleared, said the full day's work they put in Tuesday was still unusually expensive because blockade attempts kept them from leaving the plant until 10:30 p.m. and they had to be paid overtime.

The Coast Guard, which had banned boats from approaching the plant's seaward side during the protest, directed the arrests today of six men, five protesters and a stringer for the Associated Press who docked here after ferrying seven other protesters into waters near the plant. The six were released this evening on their own recognizance.