Federal officials today cleared the way for fuel loading and low-power testing at the huge Diablo Canyon nuclear power plant that has been the focus of a national antinuclear protest now expected to reach a dramatic climax.

A spokesman for the Abalone Alliance, a coalition of antinuclear groups that have been training demonstrators in civil disobedience for two years, immediately called on members to descend on the town of San Luis Obispo as soon as possible and blockade the plant. Some Abalone officials said they expect the blockade to begin in three days.

Construction on the plant, which is to provide 20 percent of northern California's total electric energy supplies, began in 1968. The plant was ready for operation two years ago, but the opening has been delayed by concerted protests from antinuclear groups that see it as a symbol of their cause and by new federal rules passed in the wake of the Three Mile Island nuclear accident in March, 1979.

Today's decision by the Nuclear Regulatory Commission's Atomic Safety Licensing Board to approve the plant's security plan set in motion a flurry of activity in San Luis Obispo, a Pacific coast community of about 34,000 about 125 miles northwest of Los Angeles.

National Guard troops have installed temporary housing, eating and shower facilities for hundreds of sheriff's deputies and highway patrolmen expected to help arrest demonstrators. The county sheriff has arranged for undisclosed temporary holding areas for those arrested, and broadcast and press crews have set up camp in local motels.

The blockade effort is one of the most ambitious ever attempted by antinuclear groups, who hope to create enough of a disturbance through peaceful protest to persuade Pacific Gas and Electric Co. and other utilities constructing nuclear plants throughout the country that the plants are not worth the trouble that ensues in plant mishaps and demonstrations.

The Abalone Alliance, which took its name after a construction mishap in 1974 killed the abalone, a marine mollusk, in Diablo cove near the plant, argues that the plant is unsafe and that radioactivity could escape into the area through a technical breakdown or an earthquake in one of the nearby faults.

The nuclear industry already is troubled by a far greater problem--escalating costs that have kept utilities from ordering any new plants and led to delays and one cancellation at plants under construction.

A Pacific Gas and Electric spokesman said the Diablo Canyon plant, which has two reactors, is to cost $2.3 billion and provide enough power for three cities the size of San Francisco. He said the plant, which could provide 20 percent of the company's generating capacity, is sorely needed because demand for power is now running as close as 94 percent of capacity and will increase.

Pacific Gas and Electric was on the verge of receiving a license to operate Diablo Canyon when the Three Mile Island accident in Pennsylvania forced a hold on all new licenses.

The full regulatory commission is to meet within 10 days to decide if the utility must delay nuclear loading further while opponents have a chance to appeal. But NRC sources have said they expect a go-ahead, and the Abalone Alliance decided some time ago to call its blockade as soon as the security plan was approved.