More than 500 customers along Commonwealth Avenue, including several local businesses, lost power yesterday morning after an above-ground transformer caught fire, according to NStar spokesman Mike Monahan.

The transformer ignited in front of Harvard Clinical Research at 930 Commonwealth Ave., between Women’s HealthWorks and the Boston University Corporate Education Center.

“The whole thing just clunked out,” said Phil Morgan, emergency coordinator for Harvard Clinical Research. “Some people heard a loud pop.”

Adrian Thomas, a student at the BU Corporate Education Center whose class was interrupted by the outage, said smoke continued to pour out of the transformer for a half-hour after it ignited.

The outage also knocked out a set of traffic lights at the intersection of Commonwealth Avenue and St. Paul Street, creating commuter confusion on both sides of Comm. Ave. Traffic signals for the Green Line were also knocked off-line.

According to Monahan, the transformer ignited at 8:52 a.m., just as many local businesses were preparing to open for the day. By 10 a.m., power for half the affected customers had been restored, according to Monahan.

Monahan said the cause of the fire is still unknown.

For local business managers, the power outage led to a short delay in day-to-day activities. But Starbucks Manager Bashar Rahabi, who said the coffeehouse lost the use of all its equipment for about 45 minutes, tried to make the most of it.

“We served pastries, coffee and tea for free for everyone in the store until we ran out of hot water,” he said.

Staff writer Joshua Karlin-Resnick contributed to this report.