A police officer was battling for his life and three more were dead after a parolee with an "extensive criminal history" opened fire at a routine traffic stop and hours later gunned down members of a police commando team searching for him.

The gunman was also killed on Saturday, capping a day of violence that the Oakland Police Department said was the worst in its history. Never before had three police officers died in the line of duty on the same day.

"It's in these moments that words are extraordinarily inadequate," said Mayor Ron Dellums at a somber news conference on Saturday night.