A series of punches left a 39-year-old man unconscious Tuesday evening after he tried to get on a Muni bus in San Francisco’s Outer Mission neighborhood, authorities said.

Officers responded at 6:23 p.m. to a report of an aggravated assault at a bus stop on the 900 block of Geneva Avenue near Mission Street. The victim was boarding the coach and had not paid the fare when he was confronted by a 25-year-old man, said Officer Robert Rueca, a San Francisco police spokesman.

The two argued before the younger man and another 25-year-old man started punching the victim several times, cutting his right lip and eyebrow and prompting him to lose consciousness, Rueca said.

“The suspects were not arguing about the refusal to pay,” he said. “They shared some words, but (it’s) not really clear what those words were.”

The suspects fled the scene and have not been arrested, police said.

First responders took the victim to a local hospital, where he is in stable condition.

SFMTA did not immediately respond to a request for comment.

Gwendolyn Wu is a San Francisco Chronicle staff writer. Email: gwendolyn.wu@sfchronicle.com Twitter: @gwendolynawu