The city’s recent surge of violence took another deadly turn Wednesday when a quarrel on an MBTA bus spilled onto the street near Dudley Square and a man in his 20s was fatally stabbed, authorities said.

Police said they received a call for a person stabbed on Warren Street in Roxbury at about 2:37 p.m. The victim, who has not been identified, was taken to Boston Medical Center, where he died.

The slaying came three days after a spate of violence in ­Boston Sunday that left nine people shot, four fatally.

Wednesday’s attack ­occurred near 65 Warren Street, police said.

Steven Turner, who works in the area and responded quickly to the scene, said the victim was alert when he was placed in an ambulance, but he had lost a lot of blood. He said the victim told him: “ ‘Get me some help. I’m getting real weak. I’m losing it.’

“This is sick. You can’t even walk the streets anymore,” Turner said.

A man who identified himself as Windy 7 said he was at a chess table on the sidewalk by the Dudley branch of the ­Boston Public Library when he heard someone yelling for help.

“He [the victim] was holding his neck, and his white shirt was red in the chest,” said Windy 7, who also said he chased the attacker.

He said he gave up the chase after a few blocks and returned to a scene he will never forget.

“He was in real bad shape,” he said of the stabbing victim.

Behind police lines Wednesday, the chair where Windy 7 said the victim sat was covered in blood, and evidence cones marked blood stains on a van and car parked next to the chess table. The intersection was closed for several hours while investigators scoured the scene.

“I’m so upset. I was so close, but I couldn’t do nothing,” Windy 7 said. After a long pause, he continued: “If I had worn sneakers I could have ran him down and caught him. I can’t believe this.”

Boston police released few details on the slaying and asked anyone with information to call homicide detectives or text or call the department’s anonymous tip numbers.

Joe Pesaturo, a spokesman for the MBTA, said in an e-mail that words had been exchanged between the attacker and the victim on the bus, but the violence occurred after the pair left the bus.

The killing in broad daylight Wednesday followed a day of bloodshed in the city Sunday.

Early that morning, a man in his 30s was fatally shot in Roslindale, and later that night four women were shot, three of whom died, while sitting in a car on Harlem Street in Dorchester.

Wednesday’s stabbing also occurred on the opposite side of Dudley Square from where 15-year-old Lance Hartgrove of Roxbury was fatally stabbed ­July 10, near a Social Security office on Malcolm X Boulevard.

The violence in the Dudley Square area is a frightening trend for Roxbury residents, and visitors, said Earl Haynes .

“People ain’t got jobs, so there’s a lot of stress,” said Haynes.

“You got to be alert. I always tell my girl to be careful, because people here snap.”

But Windy 7 said the violence is much more personal at its core, and it scars everyone who encounters it.

“They don’t respect themselves, so they sure as hell don’t respect you,” he said. “Their own parents fear these kids.

“I know I’ll be thinking long past today about this,” he said. “What a waste. He had no damn chance — that poor kid. God, I’ll never forget this.”