A massive gas explosion blew a huge chunk out of a Bronx public housing building Wednesday morning, sending bricks raining down onto the sidewalk and leaving a gaping hole in the facade. 

The 8:10 a.m. blast at the high-rise at the Mitchel Houses project on Alexander Avenue near East 135th Street left a 20-story gash along a corner of the building, and sent firefighters scrambling to the scene, officials said. 

“I heard like the building was moving,” resident Merlyno Olivo, 51, told The Post. “I heard like a big boom. I thought, ‘What is this? Is it an earthquake or something like that?’ But it started shaking quick.”

A massive collapse in the Bronx sent chunks of building plummeting to the street Wednesday morning. James Keivom

Firefighters found a collapse in the incinerator shaft caused the disaster. James Keivom

Tenant Linda Duke, 79, said she fled down five flights of stairs with the help of a younger resident — who carried her on his back.

“You want to know how loud it is? Get a firecracker and light it and listen,” Duke said. “I heard people hollering. All I could see is smoke. I couldn’t see nothing. You couldn’t see s–t.”

No injuries were reported after first responders sifted through the rubble and brought in FDNY dogs to look for survivors, officials said.

“We are extremely fortunate,” Mayor Eric Adams said during a press briefing at the scene. “At this time, we have no loss of life or injuries.

“Residents of the F and G lines in the building were evacuated, and other tenants can reoccupy the building at this time, once we ensure the safety of the structure,” the mayor said. 

He noted that Wednesday marked the first day when boilers at public housing complexes are turned on for the winter — although officials cautioned that it’s unclear if that played any role in the blast.

Firefighters from the FDNY responding to an explosion at 205-207 Alexander Ave. in the Bronx. James Keivom

“No one woke up this morning and said we want an explosion here at this building,” Adams said. “Anyone knows that when gas is involved, you want to take as much precaution as possible, and when things like this happen, you investigate.”

The section of the complex that collapsed was part of the chimney structure, NYCHA officials confirmed. They expected demolition of the remaining structure would continue over the coming days.

City Buildings Commissioner James Oddo said there are three open violations for the boilers in the Mitchell Houses, but said they are for “non-safety related defects.

“I just urge caution that we don’t jump to any conclusions that they are related,” Oddo said. “We don’t know if they are or they aren’t. We certainly will let everyone know as soon as our team has done its full assessment.”

FDNY inspecting the area at street level. James Keivom

City Fire Commissioner Robert Tucker said the explosion took place along a ventilation shaft connected to the building’s boiler, but investigators are still trying to determine the exact cause.

Con Edison, the Department of Buildings and the city’s Office of Emergency Management also responded, and the investigation is ongoing, police said.

An emergency shelter for tenants was set up at a community center on Alexander Avenue, and MTA buses “have been warranted for sheltering,” OME officials said on X.

Tenants in the affected portions of the building were later given vouchers to stay in area hotels, while other residents were allowed to return home.