SEABROOK, N.H. — It’s official: The Seabrook power plant, which became a symbol of the nation’s debate over nuclear energy, has begun regular operation.

After more than a decade of protests and huge cost overruns, the milestone was reached Sunday, utility spokesman said Ron Sher said Monday.

The plant’s operator, New Hampshire Yankee, emphasized the plant supplies enough electricity for a million New England homes; opponents still fret about the potential for disaster.

The declaration the plant is in regular operation is partly a matter of semantics. During the final weeks of testing, which were declared successful Friday night, the $6.6 billion plant’s reactor ran at full power. It has been generating at least some power for customers on the region’s power grid since tests began in May.

Seabrook Station is one of nine nuclear power plants in New England that together generate more than one-third of the region’s electricity, reducing the dependence on “extremely expensive and unreliable foreign oil supplies,” said Edward Brown, president of New Hampshire Yankee.

It took a decade to build the seaside plant, which was completed in 1986 but didn’t receive an operating license until last spring because of regulatory delays caused by the plant’s opponents.

Seabrook became the national focus of anti-nuclear activists and drew opposition from nearby residents who said the surrounding area couldn’t be evacuated safely if there was a nuclear disaster. More than 3,000 protesters were arrested at the plant over the years.

The Clamshell Alliance, which led the protests, believes state residents should view any Seabrook landmarks as setbacks said, spokesman Roy Morrison. That’s becuase the plant produces nuclear waste and has the potential to make New England uninhabitable, he said.

Court challenges of the operating license still are pending, and the Nuclear Regulatory Commission said last week it was looking into whether there were irregularities in the way the plant got its license.

Morrison said Seabrook operators made the operation official Monday to distract attention from the commission’s investigation.

The project originally called for two reactors at a cost of less than $1 billion but was scaled back to one reactor because of skyrocketing costs and lengthy constrution delays. The plant’s major owner — Public Service Co. of New Hampshire — went into bankruptcy, in part because of Seabrook costs, and is being taken over by Northeast Utilities of Connecticut, another plant owner.

The plant produces 1,150 megawatts of electricity, enough to eliminate the need for about 1.5 million barrels of oil a month, said New Hampshire Yankee.