Shuttle buses replaced Green Line service between Kenmore Square and Government Center for more than two hours amid a power outageFriday evening, marking the second time in three months that Green Line service through downtown was disrupted.

The outage began around 6 p.m., forcing some travelers to trudge through dark tunnels and disrupting travel for many fans headed to the Red Sox game at Fenway Park against the archrival New York Yankees.

Service had fully resumed by about 8:30 p.m., with delays of about 20 minutes, the MBTA said.

The outage paralyzed three Green Line trains inside the tunnels between Hynes Convention Center and Kenmore Square, according to a statement from MBTA spokeswoman Lisa Battiston. There were about 100 passengers on each train, she said.

The riders from two trains were escorted to the Hynes stop by MBTA workers, and the riders from the third train were walked to Kenmore Square, Battiston said.

The cause of the outage is under investigation, she said.

A short video clip provided to the Globe showed several dozen people walking along a narrow corridor between a Green Line trolley and the rough wall of the tunnel, then emerging past the train into the open tunnel.

The video was provided by Carter Liou, who was riding home from work on a C train when it suddenly came to a halt between Kenmore and Hynes, he said.

Liou, 23, of Quincy, said the train was crowded, with no seats available, and he was standing by the door when train stopped and some of its lights dimmed.

“There was no AC, so it was very hot, for sure,” Liou said.

The train then sat for about 20 to 25 minutes while the passengers waited to find out what was happening, Liou said.

“It felt much longer than 20 [to] 25 minutes because no one knew what was going on,” Liou said. “Nobody had any clue.”

Eventually, officials told riders to exit the train and walk back to Kenmore station on the tracks, Liou said.

At Government Center on Friday evening, rerouted travelers climbed aboard shuttle buses. Among the crowd of three or four dozen was City Councilor Kenzie Bok, who said she was on a westbound Green Line train when everyone aboard was told to disembark at Haymarket station.

“They said there was a disabled train in the tunnel ahead,” Bok said in an interview. “So then I found myself directing several loads of confused Red Sox fans towards the Orange Line, because there was no ambassador presence on the platform.”

Friday’s outage marked the second time this summer the MBTA has abruptly shut down service on the Green Line through downtown.

On June 23, both the Green and Orange lines were partially closed after support columns for the Government Center Garage were found to be “severely deteriorated” by engineers inspecting the subway tunnels, officials said at the time. Both rail lines were reopened four days later.

The Green Line’s C Branch was temporarily shut down in July, and the E Branch was shut down Saturday for construction work and is expected to reopen Aug. 21.

The MBTA is planning to shut down the entire Orange Line for repairs from Aug. 19 to Sept. 19.

Camilo Fonseca can be reached at camilo.fonseca@globe.com. Follow him on Twitter @fonseca_esq. Jeremy C. Fox can be reached at jeremy.fox@globe.com. Follow him on Twitter @jeremycfox.

Work at Boston Globe Media