Power has returned to Cambridge after an outage that plunged a large swath of the city into darkness this afternoon, including buildings at Harvard University and MIT.
NStar officials said the outage, which happened at 4:30 p.m., affected about 17,000 customers at one point. Shortly before 7 p.m., the NStar outage tracking site reported that power had been restored to all customers.
NStar spokeswoman Caroline Pretyman said NStar believes the outage was caused by a “transmission line issue” but no details were immediately available.
Cambridge police said in a tweet that power was beginning to return, but streets were still clogged after the outage extinguished traffic lights during rush hour. Police suggested people seek alternate routes.
A Globe reporter driving into Cambridge during the blackout found traffic signals out just over the Massachusetts bridge from Kendall to Central squares, with police directing traffic. Traffic was congested heading into Cambridge but moving a bit better in the other direction.
The outage prompted cancellation of a forum focused on recapping the 2012 presidential election at Harvard’s John F. Kennedy school, according to a Globe staff member at the event.
Top Obama and Romney advisers were slated to speak. Inside the school itself, classrooms were darkened but hallways were illuminated with emergency lighting.
MIT police said a “major power outage” had affected the campus. The police said they were responding to outage-related emergencies, including people trapped in elevators.
The outage caused delays on the Red Line trains from Park Street Station to Alewife Station. The MBTA said in an official tweet at 6:21 p.m. that normal service was resuming.
Boston officials also said during the blackout that they they were closing the Gilmore Bridge, which links Cambridge and Charlestown, to Cambridge-bound traffic. They also reported that traffic was clogged at Leverett and Charles circles.
Some areas had power restored early on. Around 5 p.m., in some areas of East Cambridge, near the Twin Cities shopping plaza, power flickered back on. The outage had briefly left the Cambridge Rindge & Latin hockey team in the dark as it wrapped up practice at the local ice rink.
The blackout brought back unpleasant flashbacks of a March power failure that plunged nearly 22,000 NStar customers into darkness after a fire at a Back Bay substation that was sparked when a connection between a power line and a transformer failed.
The utility installed a new transformer at the station in May, following a second power outage at the same substation that month.
The outages contributed to criticism of NStar, which had already come under heavy scrutiny – along with other utilities in Massachusetts – following a poor response last year to Tropical Storm Irene and a rare October snowstorm, which both downed power lines across the state.
Attorney General Martha Coakley, the state’s ratepayer advocate, recommended fining NStar $9.7 million for its performance in those storms. She also requested a record $16.3 million fine for National Grid, and a $4 million fine for Western Massachusetts Electric Co. However, state utility regulators have not yet acted on those requests.Sarah N. Mattero can be reached at firstname.lastname@example.org. Cynthia Needham and Roy Greene of the Globe staff contributed to this report.