CAMBRIDGE — Large swaths of Cambridge were plunged into darkness and gridlock at the height of rush hour Thursday after an NStar transmission line failure flicked traffic lights off, jolted building elevators to a stop, and forced Red Line trains to a crawl.
Most of the lights were restored by 6:30 p.m., some two hours after the outage first hit, but the line failure cast vibrant parts of the city, including Harvard University, MIT, and some shops, into an eerie darkness.
Workers and pedestrians used smartphones as flashlights to get out of buildings and guide them on darkened streets as drivers largely waited patiently while police or passersby directed traffic across darkened intersections.
About 17,000 customers were affected, an NStar official said. No injuries were reported, but police received numerous calls from people trapped in elevators, said Cambridge public safety officials.
NStar spokeswoman Caroline Pretyman said the blackout happened while the utility was conducting upgrades on a transmission line in Cambridge. While the work was being done, the main transmission line was taken out of service, and power was routed through a backup line.
NStar’s preliminary investigation showed that the second line lost power when a piece of equipment called a relay “incorrectly sensed an abnormality’’ in the electricity flow and shut the line down, Pretyman said. NStar restored power by disabling the faulty relay.
“We were able to isolate the problem, and we will investigate further,’’ she said.
Thursday’s outage was reminiscent of the March power failure that plunged nearly 22,000 NStar customers into darkness after a fire at a substation in Boston’s Back Bay. It also comes at a time when NStar and other utilities are facing more scrutiny after a widely criticized response last year to Tropical Storm Irene and a rare October snowstorm.
The outage in Cambridge caused delays on Red Line trains from Park Street to Alewife Station, as commuters trekked home.
“It took me 20 minutes to get from Harvard Square to Central Square,’’ said Tomi Uyehara, 26, of Allston. The train “didn’t move for five minutes and then it stopped in the middle,’’ he said.
Uyehara said that while the delay caught him off guard, “you can’t get upset.’’
Outside the Central Square T stop, co-workers Becca Robie, 24, and Steph Phenix, 33, said it took about an hour to get from Broadway Station to Central Square, a trip that normally takes about 20 minutes.
Robie, of Cambridge, said it was unnerving to see the lights out and smoke rising when they left the Central Square Station. It was unclear where the smoke was coming from.
“Like zombie apocalypse, post-apocalypse,’’ she said of the scene. “Thinking about walking through smoke and possibly fire to my completely dark apartment was scary.’’
Phenix, of Arlington, said they helped a woman carry her child’s stroller out of the train station because the escalators were disabled.
“When we got to Central Square and saw that all the lights were out, it was a little bit scary,’’ Phenix said.
The outage prompted the cancellation of a forum focused on recapping the 2012 presidential election at Harvard’s John F. Kennedy School of Government.
David Axelrod and Jim Messina, top figures in President Obama’s winning reelection campaign, and Stuart Stevens and Eric Fehrnstrom, two senior advisers in Mitt Romney’s campaign, were scheduled to speak for about 90 minutes in a forum moderated by David Gregory, host of the NBC News program “Meet the Press.’’
At MIT, police said a “major power outage’’ had affected the campus. The police there said they responded to outage- related emergencies, including people trapped in elevators. During the outage, all critical services operated normally, but on backup power, said MIT spokeswoman Kimberly Allen.
The CambridgeSide Galleria closed because of the outage, according to its Twitter feed.
Some areas had power restored quickly. Around 5 p.m., in some areas of East Cambridge, near the Twin Cities shopping plaza, the lights went on. The outage briefly left the Cambridge Rindge & Latin hockey team in the dark as it wrapped up practice.
Boston officials also said during the blackout that they had closed the Gilmore Bridge, which links Cambridge and Charlestown, to Cambridge-bound traffic. They also reported that traffic was clogged at Leverett and Charles circles.
A Globe reporter driving into Cambridge during the blackout found traffic signals out just over the Massachusetts Avenue bridge from Kendall to Central squares, with police directing traffic.
For NStar, the blackout comes at a time when the utility, and others, are waiting to hear about fines related to their poor response to Tropical Storm Irene and an October 2011 snowstorm.
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.
NStar has also been under a microscope in recent months because of its nearly $20 billion merger with the larger Northeast Utilities in Connecticut. The partnership, approved in April, was heralded as a deal that would give NStar more money to invest in system improvements and more crews to fix power outages.