A fire that erupted in two Back Bay transformers Tuesday evening sent a pall of black smoke over the heart of Boston, caused widespread power outages, and compelled authorities to close subway stations, block roads, and evacuate a major hotel.
Though the blaze caused structural damage only in the transformer building and no serious injuries, it paralyzed the Back Bay and South End as the evening rush hour wound to an end, forcing hotel guests to take to the streets and commuters to seek alternative ways to get home as a part of the Massachusetts Turnpike was closed to traffic.
More than 20,000 residential and business power customers lost electricity across a broad swath of central Boston, including parts of the Back Bay, South End, Chinatown, the Theater District, and Kenmore Square.
Jose Parody, 18, a Berklee College of Music freshman, said he was in the school cafeteria at 150 Massachusetts Ave. when he and about 90 other students were told to leave.
‘‘The power went out, and they told us to step outside,’’ he said.
Because streets were blocked, Parody lamented he could not reach his dorm on Commonwealth Avenue.
‘‘It just kind of feels like a movie,’’ Parody said. ‘‘All the streets are turned off and the cop lights and the sounds, the helicopter sounds.’’
Becky Ripley, 59, of Maryland was staying at the Hilton Back Bay Boston and said she was walking back to the hotel from dinner when a Boston police officer yelled for her to get inside the nearby Sheraton Boston Hotel to avoid the smoke.
‘‘It was nasty,’’ Ripley said. ‘‘It smelled like chemicals, and it burned my eyes a little.’’
Peg Schultz, 49, of Maryland, was staying at the Sheraton and said she went outside after the fire started and was confronted with an ‘‘acrid, electric smell’’ when she got to the base of the escalators in the Prudential Center.
‘‘It was a really bad smell; it was awful,’’ she said. ‘‘A smell like that is just one of those things you don’t mess with.’’
Despite initial concerns that the smoky fire might prove toxic, Boston fire authorities said there was no evidence that toxic chemicals were released.
Steve MacDonald, a spokesman for the Boston Fire Department, said Tuesday night that it was unclear how the three-alarm fire, which was reported at about 6:30 p.m., started. MacDonald said the Hilton was evacuated mainly because of concerns that the thick smoke would spread throughout the building.
Officials described the building where the fire started as a multi-story concrete structure that houses NStar transformers, which power neighborhood buildings. The building is located adjacent to the Hilton on Scotia Street.
MacDonald said firefighters had to cut off electricity because of potential danger from the affected transformers.
He said that the Hilton was evacuated and that some people were apparently trapped in elevators during that process.
Though the Sheraton was not evacuated, it did lose power, and the lobby was packed with hundreds of people Tuesday night. Many lay on the floor as hotel workers distributed water, juice, chips, cookies, and vouchers for the bar.
One person was sent to Boston Medical Center with minor respiratory problems, said Edmund Hassan, deputy superintendent for Boston Emergency Medical Services.
David Procopio, a State Police spokesman, said troopers shut down the Prudential Tunnel segment of the Mass. Pike.
Boston police spokesman David Estrada said officers shut down all or parts of Massachusetts Avenue and Arlington, Dalton, and Belvidere streets to traffic.
The Massachusetts Avenue bridge inbound from Cambridge to Boston was also closed, Cambridge police said on their official Twitter feed.
Police also ordered that Green Line trains not stop at any Back Bay stations, said Joe Pesaturo, spokesman for the Massachusetts Bay Transportation Authority. Green Line trains were bypassing Prudential, Hynes, Symphony, Copley, and Arlington stations, while the Orange Line was skipping the Mass. Avenue station. Heavily used bus routes were diverted, including the 39, which traverses the South End, Back Bay, and Jamaica Plain, and the 1, which connects Boston and Cambridge via Mass. Avenue.
Pesaturo said 12 buses were sent to assist in the evacuation of residents in the Dalton Street area.
Also, Northeastern University evacuated several student residences due to power outages and safety concerns, and said a gymnasium on campus was being set up with cots for students with nowhere to go.
Caroline Pretyman, an NStar spokeswoman, said by phone late Tuesday that the company had just gained access to the substation where the transformers caught fire, which will allow workers to assess the damage. She said about half the affected customers, from Clarendon Street to the Boston Common through Chinatown, should have their power restored by Wednesday morning.
She said it was too soon to say on Tuesday night when the remaining customers would have power back.
Copley Square was eerily quite at about 11:30 p.m. on Tuesday. In major towers, such as the Westin Hotel and the Tent City apartment building on Dartmouth Street, not a single light burned in rooms.
Dot Joyce, a spokeswoman for Mayor Thomas M. Menino, said officials reported that all T-stops were reopened before midnight, and local authorities were working with state officials to ensure the Mass Pike was open before morning. Joyce also said Boston police planned to staff every intersection without power during the Wednesday morning commute.
Michael McKeighan, director of operations for Hostelling International, at 12 Hemenway St., said that at least 200 patrons were in the 208-bed inn when the power went out.
The hostel had emergency lights on and a worker was inside with guests, he said. They did not evacuate the building.
Brian Marx, 37, of Wisconsin, was staying at the Hilton and said he saw white and black smoke outside his window after the evacuation order was announced. ‘‘We’re not going to forget this one for a while,’’ he said.
Stephen Smith of the Globe Staff and Globe correspondents Colin A. Young and Katherine Landergan contributed to this report. Travis Andersen can be reached at email@example.com. Follow him on Twitter @TAGlobe. Zachary T. Sampson can be reached at firstname.lastname@example.org.