The Fairfield Police Department received calls from a home in the 3500 block of Valley View Court saying they were having a party and that it had turned violent.
One caller said there was a large fight in her backyard and people were using broken beer bottles as weapons, according to police.
Officers responded and located at least 40 people in and around the home. At least three people were taken to a nearby hospital where they were treated and released.
Police stopped a vehicle that was leaving the home. As the driver was being arrested for driving under the influence of alcohol, police found a semi-conscious person in the back seat with serious head trauma.
The injured person was transferred to UC Davis Medical Center and is on life support, according to the Police Department. Everyone in the vehicle was from San Leandro.
Anyone with information regarding the fight is asked to contact the Fairfield Police Department at (707) 428-7300.