Flames erupted in the electrical room between floors four and five, and between the fifth floor and the roof, FDNY Assistant Chief David Simms said during a news briefing. 

“This was a deep-seeded fire involving a lot of electrical equipment,” he said.

One firefighter suffered a minor injury. All theater workers were safe and accounted for, authorities said.

The theatre has hosted “The Book of Mormon” since 2011, though no shows were scheduled for Monday.

Simms said the building would be “out of service pending repairs” and the city Department of Building was expected to investigate.

“There’s been substantial damage on the fourth floor and the electrical room, which contains lighting equipment and the hanging chandeliers,” he said.

Performances at the theater were canceled at least for Tuesday and Wednesday, a rep for the acclaimed show said in a statement.

“The show will continue to work with theatre owners, ATG Entertainment, the FDNY, and other relevant parties to assess the damage resulting from a fire at the Eugene O’Neill Theatre this morning,” Book of Mormon and ATG Entertainment said in a joint statement through a publicist.

Customers affected by the cancellations will either get to exchange tickets or receive refunds.

Tourists staying at a hotel next door to the theatre said they were forced to flee while firefighters battled the blaze.