About 30 more NStar customers across six or seven buildings near the intersection of Beacon Street and Massachusetts Avenue lost power Sunday when flames burst from a manhole at about 8:45 p.m., officials said.

Bill Zamparelli, a spokesman for NStar, said the fire was not related to the major outage that hit the Back Bay Sunday morning, even though the fire started in the same network of circuits.

After power was restored in a massive days-long Back Bay power outage in 2012, a sudden surge in demand for power caused underground cables to fail and sent manhole covers soaring into the air. Zamparelli said the cause of Sunday’s underground fire was not known, but that manhole fires are usually caused by a cable fault or a shortage.

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Steve MacDonald, a spokesman for the Boston Fire Department, said firefighters used an extinguisher to put out the flames after NStar shut off power to the equipment in the manhole near 461 Beacon St.

Zamparelli said NStar crews continued to work late Sunday night and hoped to complete repairs overnight.

NStar trucks and crews surrounded several other manholes in the area Sunday night.

“Normally when we have a fire in the underground they’ll go around the area to see if there’s any other damage,” Zamparelli said.

Trish Fuller said she was standing outside her apartment building on Beacon Street about 10 feet from the manhole when she saw smoke pouring from the manhole, then lights inside buildings up and down the block violently flickering.

Fuller said she had seen a manhole explode once before, years ago on Newbury Street, and expected that scenario was about to unfold again. She yelled out to a man who was about to walk right over the manhole.

“I said ‘stop, that’s going to blow up,’” Fuller recalled.

The man stopped. Flames shot out feet in the air once, and then again a split second later.

Afterward, she said both she and the man were shaking and the man, whom she had never met before, gave her a hug.

“It was terrifying,” she added. “Especially after those Marathon bombings.”

Mike Griffith, 24, lives a couple doors away on Beacon Street and his apartment also lost power. Unable to run air conditioning, he sat outside Sunday night to cool off. Before the fire, lights were flickering in his apartment, he said.

“When I first came out of my building to see what was going on [men wearing yellow vests and working in the street] told me to go back inside,” Griffith said.

Shortly afterward he heard a loud noise and the air smelled like burnt plastic. He later saw smoke billowing from the manhole.

“A lot of people came outside wondering what’s going on,” he said.

He had been looking forward to watching the finale of the popular HBO series “Game of Thrones,” which was scheduled to air about a half hour after the power went out. Shortly before 11 p.m., when the episode was scheduled to air again, Griffith held a bit of hope that the power might be restored in time for him to catch the show.