Disturbing video obtained by The Post shows a man allegedly pushing his grandmother out a first-floor window — leaving her bleeding from her face on the pavement — after slashing his mother repeatedly in the Bronx, cops said.

The footage shows the distressed 88-year-old hanging out of a window at 1812 Clay Ave. around 7 a.m. Friday, as her daughter, 61, stands on the sidewalk.

The younger woman, wearing a white shirt soaked in blood, appeared to be speaking with the grandmother.

After a short time, two hands can be seen shoving the older woman from the window.

Her body slowly edges out of the opening before tumbling onto the pavement.

She then rolls over, and appears to be bleeding from her nose and mouth, the clip reveals.

Ernest Thompson, the building’s superintendent, said he came outside to see what the commotion was and saw his tenants in distress.

“I ran out and I saw her neck and side mutilated,” he said. “So I told her, ‘Wait for me. I’m going to get compression to put on your wounds.’

“Before I had a chance to get a few feet away her son was pushing his grandmother directly out of the window,” he said.

He said he grabbed the grandmother and told her not to move and that he was getting help. By then, the cops had arrived.

Both women were taken to St. Barnabas in stable condition. They weren’t identified by police.

The son, Randy Sierra, 38, was found inside with a self-inflicted stab wound to his chest. He was taken to the same hospital in critical condition and has been charged with assault, cops said.

The grandmother told cops she opened the window to yell for help because her grandson was scaring her. Before he pushed her, she told police he was “poking her” as she hung out the window.

“I was seven steps away,” the building super said. “I could have caught her if I turned around fast enough.”