Hundreds of others were evacuated from the stations, with bus service being brought in for riders.

‘‘At this point, it’s unclear’’ what started the three-alarm electrical fire, Pesaturo said. He said cables were found melted together in the Downtown Crossing tunnel.

Pesaturo said that the Red Line will be closed Friday morning in the area of Downtown Crossing. He said officials ‘‘don’t anticipate’’ Green Line closure, and the Downtown Crossing section of the Orange Line is uncertain.

John Callaio, 24, of Dorchester, said he was taking the south-bound Red Line home Thursday night when an announcement told passengers to get off at the Downtown Crossing station. Seconds later, he saw smoke, and T employees directed him to get outside.

‘‘The smoke was filling up the whole station as we were leaving,’’ Callaio said.

John Gill, the deputy superintendent of Boston EMS, said people complained of burning sensations in their throats and coughing, from the smoke. He said everyone was in ‘‘very stable condition.’’

Several people outside the Downtown Crossing station Thursday night said it was filled with white smoke when they got off the train.

John Guy, of Dorchester, was coughing repeatedly as he described the chaotic situation. He said that he had to hold on to someone he did not know to find his way out of the station.

‘‘The smoke was so thick, you couldn’t even see,’’ he said. Guy said that when he got out of the station, he doubled over in pain.

Dorchester resident Evelyn Vazquez said that the smoke came into the station incredibly quickly. ‘‘It was like two seconds and the whole thing was white,’’ she said.

Despite the apparent severity of the incident, some passengers said there was confusion initially as T workers told them to remain on the train as the station filled with smoke.

Kevin Nguyen, of Dorchester, said that at first, a T worker said over the intercom to stay on the train because the smoke was not coming from the trains.

‘‘She said, ‘everybody relax, and stay on the train.’ But a couple minutes later the doors opened and people spilled out into the station.

One passenger, Mattapan resident Murdock Cadogan, described how a woman he was helping passed out. ‘‘She was just coughing’’ and throwing up, he said.

Shannon Hebert, 27, of Malden, said the smoke came on fast as she was getting off the Red Line and she fled from Downtown Crossing.

‘‘The smoke was almost going as fast as me,’’ Hebert said. ‘‘I was hacking, coughing.’’

She said she smelled smoke when she got off the train, and saw several MBTA employees down in the track.

Passenger Juan Garcia, of Belmont, said the smoke was so dense that he could not see his hand in front of his face. ‘‘The smoke was like a storm,’’ he said.

Tara Harris, of Dorchester, said it was chaotic when people ran from the station. ‘‘It was like a mad rush, you had to feel your way out.’’

Late Thursday night, Pesaturo said T workers were just being let down into the Down Crossing station to assess the damage.