Officers responded to a report of a possible vehicle-versus-pedestrian accident in the area of Park Lane and McGregor Avenue at about 7:10 p.m., police said.
When officers arrived, they learned that a 47-year-old man was crossing the street when the suspect car nearly struck him, according to police.
The pedestrian and the driver exchanged words, then the driver stopped his car, got out with a blunt object similar to a pipe and struck the man in the head, police said.
The suspect - described as a man in his 20s with a thin build and short hair - fled in his car heading north on Park Lane. His car is described as a dark-colored, late 80s or early 90s four-door sedan, similar to a Buick, according to police.
The victim was transported to a hospital to be treated for a possible skull fracture and other facial injuries. As of Sunday, he remained in critical condition, police said.
Anyone with information about the incident is asked to call Petaluma police at (707) 778-4372.