About 600 demonstrators launched a blockade against the Diablo Canyon nuclear power plant today, but failed to disrupt preparations to load nuclear fuel.

Nearly 200 demonstrators blocked the closed main gate, some climbing over by using ladders. Approximately 70 demonstrators inside the gate sang and chanted for about three hours, then were led by sheriff's deputies to a school bus and taken to a holding facility inside the plant. A workman sawed the ladders in two.

Before nightfall, the sheriff's office said 357 had been arrested for trespassing. Most were demonstrators who had hiked to the plant's interior fences some distance from the main gate. Later, about 120 who had sat down outside the gate were arrested and taken into the plant area.

The earlier group had hiked overnight through the rugged Pacific Coast hills that surround the twin reactor plant, about 150 miles northwest of Los Angeles. Perhaps 400 demonstrators converged on the plant's fences from several directions today, including several arriving on the rocky shore in rubber motorized rafts.

Although members of the anti-nuclear Abalone Alliance said they planned more organized trespassing and gate blocking Wednesday, the relatively small turnout at what was to have been the key U.S. anti-nuclear protest of the year appeared to make it unlikely that more such protests will be attempted soon.

The American nuclear industry is teetering from a series of financial and regulatory blows, but carefully organized mass demonstrations like this seem unable to attract more than a few hard-core activists, particularly when arrests are a threat.

"It's the quality, not the quantity," said Sophie Fishman, 39, a homemaker from nearby San Luis Obispo, as she sat on the asphalt road in front of the plant's main gate. She said her son, Davey, 11, encouraged her to protest what she called the threat of "nuclear waste products which could pollute our country."

The demonstration has almost no chance of meeting its original goal, stopping the planned loading of nuclear fuel, because the fuel has been kept on the plant site for several years, and workers responsible for loading it have been living in temporary quarters there.

Several demonstrators said today that even though they may not be able to stop the plant's operations, they hope their actions will convince other Americans that nuclear power is wrong.

A small army of reporters crowded the main gate today and posed a greater hindrance to traffic than did the demonstrators.

Greg Wellish, 26, a truck driver from northern California, lifted a full pack onto his back as he entered the woods of Sea Canyon for his trek to the plant fence Monday night.

"The people who say nuclear power is safe better pray they are right," he said. About 50 others, including two 10-year-old boys, joined him in the 12-mile hike on a beautiful night of rolling fog and a full moon.

Sgt. Leon Cole of the San Luis Obispo County Sheriff's Department said deputies posted inside the plant's fence plan to arrest all demonstrators who intrude on the plant property that extends outside the fence. Several, however, may stop in areas difficult to each by Jeep or horseback, and "It may be a while before we can get them," he said.

Hikers arrested today were brought into the plant through gates in the eight-foot double fence and kept in a temporary chain-link holding area. They are to be taken later to the Califonia Men's Colony, a minimum-security prison near the sheriff's office, 12 miles east of the plant.

About 1,500 people have gathered at a temporary camp site on unused farmland for the protest, which has been inspired in part by the vocal opposition to Diablo Canyon by California Gov. Edmund G. (Jerry) Brown Jr.

Brown shouted "No on Diablo Canyon!" at a rally near here in 1979, and still supports court and administrative efforts to stop the fuel loading, but he has told the demonstrators that they will be arrested for trespassing or blocking roads.

The charges are mostly misdemeanors carrying maximum fines of $500 and short jail terms, but boats which defy a Coast Guard ban on sailing near the plant could be confiscated and their owners fined as much as $50,000 or given multi-year prison sentences.

The demonstration, after a call for protesters went out last Wednesday, has been slow in starting and low in numbers, partly because organizers insisted that all participants be trained in nonviolence and make decisions through a consensus of so-called affinity groups.

The sheriff's department and the Pacific Gas and Electric Co., owner of the plant, have handled the demonstration carefully so far. One highway patrolman carried on a friendly chat with anti-nuclear first-aid teams about his plastic handcuffs today.

Cole said 25 people who landed on the ocean side of the plant asked not to be arrested, but simply escorted off plant property. Their request was granted.

Blockade organizers said 15 small boats sailed just short of an area under Coast Guard restriction near the plant. Some demonstrators went ashore from dinghies.

The Nuclear Regulatory Commission is expected to meet Monday to give final approval for fuel loading and low-power testing at the plant. Diablo Canyon, which could provide 20 percent of northern California's electricity, is not expected to open before next year. It was essentiallly completed five years ago, but has been held up by court and administrative challenges to its license ever since.