Police are investigating a brutal beating that left a man with life threatening injuries, including a fractured skull, in San Francisco’s Portola neighborhood early Friday morning, authorities said.

Officers responded to reports of the attack around 12:45 a.m., said Officer Albie Esparza, a police spokesman, near the corner of San Bruno Avenue and Bacon Street.

Investigators determined the victim, a 51-year-old man, had been waiting at a bus stop when he was attacked for no discernible reason by an unknown assailant wielding a stick, Esparza said.

The victim suffered a fractured skull, bleeding on the brain, a punctured lung and broken ribs, Esparza said, and was being treated at a local hospital.

There was no apparent motive, Esparza said, and no description of the attacker.

Anyone with information about the attack can contact investigators at (415) 575-4444.

Kale Williams is a San Francisco Chronicle staff writer. E-mail: kwilliams@sfchronicle.com Twitter: @sfkale