In a chilling incident in Harlem, a mother was brutally stabbed by her ex-boyfriend, who lay in wait for hours before launching his vicious attack, according to sources and building staff.

The 47-year-old woman, a mother of four, pleaded for mercy, calling out her attacker’s name while he repeatedly stabbed her. The horrifying scene unfolded in a hallway of a six-story apartment building on West 129th Street early Monday morning, as reported by law enforcement and building workers.

At approximately 4:25 a.m., a building porter discovered the woman, covered in blood and barely clinging to life, near the third-floor elevator. He immediately alerted authorities by calling 911, police and sources confirmed.

Lyron Middleton, who was cleaning the staircase at the time, described the moment he came across the resident, noting she has a teenage son with autism. “As I approached, I saw the blood,” Middleton recounted to The Post. “At first, I thought it was just someone lying on the floor.”

“I saw the blood as I got up close to her,” Middleton told The Post. “I thought it was just someone lying on the floor.”

Responding EMS workers rushed the critically injured woman to Mount Sinai Morningside hospital, where she died from her wounds, cops said. 

Middleton said his boss tried to show him surveillance video that captured the savage attack, but he couldn’t bear to watch it and only listened to the audio.

“She was yelling out someone’s name as it was happening,” he said of the footage. 

“She said, ‘Luis, Luis, Luis. Luis stop, Luis stop,’” he said. 

A police source who viewed the video said the enraged attacker knifed the victim in her face, head and neck.

“He must have stabbed her 50 times,” the source said.

It appeared the assailant sat on the building staircase for two hours — beginning at 2 a.m. — before he pounced when she showed up at 4 a.m., Middleton said. 

“He must have been waiting for her,” he said.

A male person of interest — described as the victim’s ex-boyfriend — was in custody by the afternoon, but was not immediately charged, sources said.

A building superintendent told reporters that the victim had four kids, two who live with her and two who are older and live in New Jersey. 

“I don’t know how did this happen, because they are not a problem person, any one of them. They are nice [people], like our family,” the worker said.

A rattled neighbor described the murder as “a sad situation knowing she had children.”

“I just see the whole damn police out here this morning,” the resident said. “I was shocked. That has never ever happened in this building and I’ve been here for 10 years. I’m really shocked.”