Update 10:40 p.m.
NStar estimates there are least 20,000 customers in the dark in the Back Bay, South End, Chinatown, the Theater District and Kenmore. Power will likely not be restored until sometime tomorrow morning.
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Update 9:28 p.m.
Firefighters have contained the blaze, but police will remain on duty as the area is cleaned up and damage is assessed. People are encouraged to keep their distance.
Mayor Menino is on the scene, and power outages could last until tomorrow morning, the City of Boston said. NStar says 20,000 customers have been affected.
Boston Medical Center confirmed they treated one person for minor smoke inhalation, but no other injuries have been reported.
Update 8:22 p.m.
Firefighters are struggling to put out what is now a four-alarm fire caused by two 115,000 volt transforms that exploded in the Back Bay Hilton parking garage. The Hilton was evacuated at 7 p.m.
The thick, black smoke was originally thought to contain chemicals, but officials now say it came from oil and is no longer considered toxic. Back Bay residents and businesses are asked to remain inside, and to keep the windows shut.
Green Line stations from Symphony to Arlington are all closed and power is down in most of the Back Bay. Firefighters have received a few reports of people stuck in elevators.
There's no estimate of how long it will take NStar to restore the power, or when the fire will be extinguished.
Update 8 p.m.
Fire upgraded to a four-alarm. Officials advise all residents and those in the area to remain indoors and keep windows closed.
Update 7:45 p.m.
Symphony, Hynes, and Mass Ave MBTA stations all closed.
People on the T platforms in the closed stations will be allowed to board trains, but will not be allowed to return to the street.
Power has been shut down throughout Back Bay.
Firefighters responded at 6:27 p.m. to a three-alarm electrical fire in the Back Bay Hilton parking garage that's producing a thick, black toxic smoke from two large 115,000 volt transformers that exploded.
The hotel was evacuated at 7 p.m. and Boston Police area asking people to seek alternate routes around Dalton Street for safety reasons.
At 7:17 p.m. @AlertNewEngland tweeted:
"MBTA PD advising their officers that the smoke from the Dalton St fire is toxic, can't be filtered by their masks."
@Boston Fire also tweeted:
"All ladders trucks were pulled away. NSTAR crews working with Chief. Still thick, black heavy smoke."
The two transformers are adjacent to but not connected to the Hilton.