The gunman started shooting after entering the backyard of a south Fresno home where a gathering of family and friends was watching a football game after 6 p.m., police said, and neighbors flooded 911 dispatchers with calls for help.
The suspect fled the scene and police were combing the neighborhood for witnesses and possible security camera footage.
Police did not release the number of dead at a news conference late on Sunday, but said there were multiple deaths among the ten known to have been shot in the city, about 200 miles (322 km) north of Los Angeles.
Others at the party may have driven themselves to hospital, police said, adding that some victims were taken to a nearby community medical center with gunshot wounds.A police spokesman was not immediately available to Reuters for further comment.