The crash was reported at about 5:45 a.m. at the intersection of Third and 22nd streets, where the T-Third Muni vehicle and a police car collided, Muni spokesman Paul Rose said.
The Muni vehicle broadsided the police car but was not going at a high speed, said fire department spokeswoman Mindy Talmadge.
The two officers in the police vehicle were transported to the hospital, as were the Muni operator and a passenger in the light-rail vehicle, Talmadge said.
All of the injuries are considered minor and not life-threatening, she said.
Rose said Muni and police officials are "still investigating the specific cause of the incident."
Both vehicles remained in the intersection as of 7:30 a.m. while investigators inspected them, according to an employee at the nearby Just For You Cafe.
Rose said the intersection was closed and Muni will be running bus shuttles along the T-Third line to supplement service until further notice.