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.