A 30-year-old man confessed today to being the subway psycho who “launched” an innocent straphanger into tracks, where he was killed by an oncoming Q train, law enforcement sources told The Post.

The suspect, Naeem Davis, was being questioned today in Manhattan, in connection to the grisly death of Ki Suk Han, 58, yesterday afternoon. The man was picked up on 50th Street near Seventh Avenue by a transit police captain, who was on a coffee break at 1:30 p.m. and ran over to grab him.

Davis confessed to shoving Han into tracks, though he’s not been formally charged yet, law enforcement sources said.

Han, of Elmherst, Queens, desperately tried to scramble back to the platform as onlookers screamed, shouted and frantically waved their hands and bags in a bid to get the downtown Q train to stop at around 12:30 p.m.

The man being questioned by cops was identified as a 30-year-old street vendor from Queens, according to law enforcement sources.

Post freelance photographer R. Umar Abbasi — who had been waiting on the platform of the 49th Street station — ran toward the train, repeatedly firing off his flash to warn the operator.

“I just started running, running, hoping that the driver could see my flash,” said Abbasi, whose camera captured chilling shots of Suk’s tragic fight for his life.

The train slowed, but a dazed and bruised Han still wound up hopelessly caught between it and the platform as it came to a halt.

A shaken Abbasi said the train “crushed him like a rag doll.”

“It’s one of those great tragedies, it’s a blot on all of us,” Mayor Bloomberg said today. “And if you could do anything to stop it, you would. But the good news is it happens phenomenally rarely.”

Dr. Laura Kaplan, a second-year resident at Beth Israel Medical Center who was also on the platform, sprang into action, taking off her coat, grabbing her stethoscope and rushing over to try an administer CPR with the help of a nearby security guard.

“It was terrifying, but you run on adrenaline,” Kaplan told The Post. “There was no pulse, never, no reflexes.”

“I heard what I thought were heart sounds,” she added. “We started compressions, which is half of CPR. We were unable to perform rescue breathing [the other half of CPR] because there was blood coming out of his mouth. He wasn’t in the right position [for full CPR] and there was just no way to get him out of there.

“It was apparent there was not much I could do — but you can’t not do something, you have to try.”

Kaplan said she had been sitting on a bench waiting for the train and heard people arguing but did not see Han thrown onto the tracks.

“I looked up and briefly saw the man standing up vertical along the tracks, and that’s when the train hit him,” she said.

“He flipped, vertically, a number of times. People were shouting and yelling when it happened, but then people ran the other way,” she said.

Han, who lived with his wife and college-age daughter in Elmhurst, was taken to Roosevelt Hospital, where he was pronounced dead.

The horrific drama unfolded after Han approached a crazed Davis — who police sources described as a panhandler and witnesses said had been harassing and cursing at straphangers — on the southbound platform and tried to calm him down.

“There was a confrontation before. It was not unlike a car crash where people focused after the fact, rather than what happened before with the confrontation,” NYPD Commissioner Ray Kelly said today.

“We have various reports. We have three calls we’re checking on. Witnesses say the individual was talking to himself prior to pushing the victim. Some sort of confrontation takes place.”

As other riders congregated toward one end of the platform, Han and the man were about 100 feet away from them.

“He went up and tried to calm him down, saying, ‘You’re scaring people,’ ” a law-enforcement source said. “The emotionally disturbed guy just started screaming and cursing, saying, ‘You don’t know me! You don’t know who I am!’ ”

As the train’s arrival was announced over the loudspeaker, the attacker “just grabbed [Han] and launched him — just threw him — straight onto the tracks,” a witness said.

The killer then grabbed a paper coffee cup he used to collect change — which he’d put down before the assault — and fled.

Abbasi recalled, “Out of the periphery of my eye, I just saw a body flying, flying through the air.

“People started waving their hands, anything they could find. They were shouting to the man in the tracks, “Get out! Get out of there!’ ”

Han barely missed the third rail, cops said, and looked stunned as he sat up in the track bed as the train approached before scrambling to his feet.

At one point, Han stood in the tracks and looked directly at the oncoming train lights.

“The most painful part was I could see him getting closer to the edge. He was getting so close,” Abbasi said. “And people were running toward him and the train.

“As I was running toward the train, the man I believe pushed him ran the other way, and I heard him say, ‘Goddamn motherf–ker.’

“I didn’t think about [the perp] until after. In that moment, I just wanted to warn the train — to try and save a life.”

One witness said Han was dragged 10 to 15 feet.

The train’s operator was treated for shock and brought out of the station in a wheelchair, wearing an oxygen mask.

“He’s traumatized,” a transit source said.

Abbasi said the driver saw his camera flashing but told him he couldn’t stop the train fast enough.

Han’s devastated wife said she and her husband had quarreled before he left the house at around 11 a.m. and headed for Manhattan.

She told cops he’d been drinking, and one witness claimed he was the aggressor on the platform, law-enforcement sources said, adding that authorities found a bottle of vodka on Han afterward.

“We had a fight,” the widow said through tears. “I kept calling him and calling him to see where he was, but he didn’t answer.”

Additional reporting by Jamie Schram, Jennifer Fermino and Laurel Babcock