As the mayhem unfolded, the train pulled into the 25th Street station, and straphangers were evacuated to the R train across the platform, where some got off at the next stop and others were rushed to hospitals.
So far, the NYPD has not confirmed any fatalities stemming from the incident.
Footage taken from the chaotic scene showed screaming passengers spewing out onto the platform as soon as the train doors opened and clouds of smoke billowing out.
Graphic photos on social media showed the injured lying on bloodstained subway platform floors.
A straphanger who was on the train when the violence broke out told The Post there were so many rounds fired off, she “lost count.”
“There was like, lots of them. I don’t even know how many,” said the woman, who only gave her first name, Claire.
She said she saw the suspect — who was described as a 5-foot-5 man, around 170 pounds and wearing an orange vest and gas mask — drop “some kind of cylinder that sparked at the top.
“I thought he was an MTA worker at first because I was like, I didn’t like pay too much attention, you know, ‘You’ve got the orange on,’ ” she said.
The NYPD’s bomb squad was on the scene investigating, and authorities were scouring MTA surveillance to try to identify the suspect.
Cops asked reporters at the scene to be clearly identified so the suspect doesn’t attempt to blend in with the crowd.
The injured were taken to NYU Langone of Brooklyn, Maimonides Hospital, Kings County Hospital and Methodist Hospital, officials said.
Mayor Eric Adams has been briefed on the incident, a spokesman said, and the NYPD is slated to have a news briefing from the scene at about 11:30 a.m.
“While we gather more information, we ask New Yorkers to stay away from this area for their safety and so that first responders can help those in need and investigate,” the spokesman said.