NEW YORK — A man has been arrested after allegedly killing three people in an apparently unprovoked stabbing in Manhattan, authorities said.

Ramon Rivera, a 51-year-old homeless man, was charged with three counts of first-degree murder as a serial killer after he was taken into police custody on Monday.

He was found with blood on his clothes and is believed to be homeless and living at the Bellevue Men’s Shelter on East 30th Street.

The first victim, a 36-year-old man identified as Angel Lata Landi, was fatally stabbed in the abdomen at 8:22 a.m. Monday in an unprovoked attack at the construction site where he was working on West 19th Street, he said. NYPD.

About two hours later, a 68-year-old man was stabbed multiple times on East 30th Street, police said. The identity of the deceased has not been revealed, pending notification of the family.

The third victim, a 36-year-old woman, was picked up around 10:55 a.m. at 42nd Street and First Avenue, police said. She had multiple stab wounds to her chest and arm and was taken to New York Presbyterian/Weill Cornell Medical Center, where she later succumbed to her injuries.

An Uber driver witnessed the third attack and followed Rivera down First Avenue, yelling out the window to pedestrians for help.

“I’m walking around with him, I’m like ‘drop it man, it’s not worth it,'” said witness Baha Biten. “I told him ‘drop the bag, there will be police ahead.’

Veteran NYPD officer Robert Garvey was there.

“I just heard a commotion, went outside, heard what people were saying and they reacted,” Garvey said.

He stopped Rivera, dropped his bag and found not one, but two knives. One of them was stained with blood. He was arrested at East 46th Street and First Avenue.

“If Officer Garvey hadn’t been there, these random acts of violence, this person probably would have continued those random acts of violence. So, he saved the lives of fellow New Yorkers,” Adams said.

The 51-year-old suspect — who was staying at the Bellevue Men’s Shelter on East 30th Street — appeared to choose his victims at random, police said.

He was caught near East 46th Street and 1st Avenue, police said.

“He just walked up to them and started attacking them with the knives,” Detective Chief Constable Joe Kenny told a news conference.

Surveillance video obtained by Eyewitness News shows the man donning a hoodie, cap and gloves, then pulling a large knife out of his backpack, hiding it in his shirt before leaving.

The suspect has eight prior arrests, according to law enforcement, and is believed to have serious mental health issues, Mayor Eric Adams said.

“There is a real question as to why he was on the street,” Adams said.

Two bloody kitchen knives were recovered, police said.

“These random attacks left two innocent victims dead and another fighting for her life,” Kenny said. “We have one suspect in custody and are not looking for others at this time.”