LOS ANGELES (AFP) - A gunman killed three people in California on Tuesday, including at least one execution-style, in a series of carjackings before taking his own life, police said.

At least two other people were injured as the gunman fired randomly on other vehicles while driving a stolen car on the freeway, before he shot and killed himself in the city of Orange, south of Los Angeles.

The first killing appeared to be a woman found dead in her home in Ladera Ranch, after neighbors reported hearing gunfire in the early hours. The suspect, a man in his 20s, then fled in an SUV.

"Multiple incidents" then occurred as the man drove to nearby Tustin, about 56km south-east of Los Angeles, said Tustin police department spokesman Paul Garaven.

In one carjacking, the suspect got out of his vehicle, and confronted a motorist in a BMW.

"He orders him out of the vehicle, walks him to the side of the curb, and executes (the) victim," said Santa Ana Police Corporal Anthony Bertagna, updating reporters on the early morning shooting spree.

Orange County sheriff's spokesman Jim Amormino said four people were confirmed dead including the gunman.

"With each carjacking there was a shooting involved," he said, adding that there was no active pursuit of the suspect's vehicle, until it was located in Orange, where the gunman took his own life.

"He stopped the vehicle and got out and shot himself," he said.

The total number of injured was unclear, as further reports emerged of people shot at while driving on the freeway.