This story was reported by David Abel, Martine Powers, and Travis Andersen of the Globe staff and Globe correspondent Zachary T. Sampson. It was written by Andersen.
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 Theatre 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.
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, 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.
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., had started. The Hilton was evacuated mainly out of concern that the thick smoke would spread through the building, MacDonald said.
Officials described the building where the fire started as a multistory, concrete structure that houses NStar transformers, which power neighborhood buildings. The building is 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 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.
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 closed 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 bypassed Prudential, Hynes, Symphony, Copley, and Arlington stations, while the Orange Line skipped 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 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 at about 11:20 p.m. Tuesday that the utility had just gained access to the substation where the transformers caught fire, which will allow workers to assess the damage. She said about half of 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 when the remaining customers would have their power back.
Copley Square was eerily quiet at about 11:30 p.m. on Tuesday. In major towers, such as the Westin Copley Place Hotel and the Tent City apartment building on Dartmouth Street, not a single room light was on.
Dot Joyce, a spokeswoman for Mayor Thomas M. Menino, said officials reported that all MBTA stops were reopened before midnight, and authorities were working with state officials to ensure that the Mass. Pike opened. 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.
Brian Marx, 37, of Wisconsin, said he saw smoke outside his Hilton window after the evacuation order. “We’re not going to forget this one for a while,’’ he said.