PLYMOUTH, Mass. (AP) _ The Pilgrim nuclear reactor began low-power testing Friday after receiving final federal clearance nearly three years after being shut down and labeled one of the country's worst-run power plants.

Boston Edison Co., the owner, said the restart began about 6 p.m. with the withdrawal of control rods in the reactor.

Opposition to Pilgrim's restart has been intense, with state officials pledging to do everything they can to keep the plant from operating and protesters planning to demonstrate at the plant gates Saturday.

The Nuclear Regulatory Commission cleared the way Friday for low-power testing of the 670-megawatt reactor for up to 5 percent of its capacity.

The restart began a test program intended to take the plant gradually to full power in about four months, said Ralph Bird, Boston Edison senior vice president for nuclear power.

Bird said the utility would continue to work with state and local officials to complete and improve an emergency evacuation plan for towns around the plant. State officials have called the plan inadequate despite revisions, and they also say the plant should not reopen until a study of leukemia rates in the population surrounding the plant is complete.

''We will do whatever we possibly can legally, through the attorney general, to ensure that the plant doesn't operate,'' said Martie Barnes, a spokeswoman for Gov. Michael S. Dukakis. ''We still believe the plant is not ready for restart.''

At the time Pilgrim was shut in April 1986, the NRC called the plant ''one of the worst-run'' nuclear plants in the country.

Boston Edison has spent about $200 million since the shutdown to replace nearly all of the plant's top management, work on mechanical problems and fight skepticism by the NRC and a growing anti-nuclear movement among nearby residents.

If the plant's restart schedule is followed, consumers can receive power from Pilgrim when it hits 10 percent operating capacity sometime next month.

By the end of January, the plant should be at about 25 percent of power, producing 100 megawatts daily, or enough electricity to power 160,000 homes, Boston Edison spokeswoman Elaine Robinson said.

Massachusetts Citizens for Safe Energy, an anti-nuclear group, Friday called the reopening ''a slap in the face to local and state government officials and a direct threat to thousands of people living near the Pilgrim nuclear plant.''

''There are increasing signs that Pilgrim has caused leukemia in nearby residents,'' the group said in a statement.

Boston Edison officials said there is no evidence that radiation from the plant has caused any cancer deaths.

Citizens for Safe Energy also said the plant should not be brought to full power because ''Pilgrim's history of management and safety failures has continued despite NRC and Edison claims to the contrary.''

Said Robinson: ''We're confident that we're proceeding in a very safe and responsible manner and we wouldn't procede if we didn't think it was safe. It's important to realize how many of us live right here with our families.''

In October 1987, Dukakis and Attorney General James Shannon, citing evacuation and safety concerns, asked the NRC to revoke Pilgrim's operating license.

That request was formally denied Thursday in a letter from Thomas E. Murley, director of the NRC's Office of Nuclear Reactor Regulation.

In a letter dated Friday, NRC Regional Administrator William T. Russell gave permission for the plant to operate, saying the technical issues raised in the shutdown had been resolved.

The Pilgrim plant must undergo another review by the NRC before it is granted permission to operate at full power, a process that could take months.