Police said they arrested two suspects and then conducted a helicopter manhunt for four other suspects.
The incident began Tuesday at 2 p.m., when as many as six people robbed a person near a restaurant, Sgt. Tommy Thompson said.
Two of the suspects then got into a car, which collided with a pickup truck at a busy intersection as a police officer chased it, KPNX-TV, Phoenix, reported.
"Two individuals got out of that vehicle (and) he began to give them orders at gunpoint but unfortunately those two individuals were able to overpower him," Thompson said.
The truck's driver, who was later identified as a 63-year-old volunteer member of the sheriff's posse, attempted to aid the officer.
One of the suspects grabbed the officer's gun and shot the victims.
The off-duty volunteer, who was not identified, is in critical condition at the hospital.
The officer, whose name was also not released, was taken to the hospital and underwent surgery, KPNX-TV reported.