The attacks took place in the coastal city of Half Moon Bay. The victims were all Chinese-American farm workers. 

Suspect Zhao Chunli, 67, was arrested after driving to a police station.

It comes as the state mourns the death of 11 at Monterey Park - about six hours southeast of Half Moon Bay - during Lunar New Year celebrations. 

And just over a week ago, six people, including a teenage mother and baby, were killed at a property in Goshen, central California.

California Governor Gavin Newsom tweeted that he was at a hospital meeting with victims of the earlier mass shooting when he was pulled away to be briefed about the second attack, describing it as "tragedy upon tragedy".

The latest bloodshed to hit the state occurred at two farms around the Half Moon Bay community.

Four victims of the shooting attack were discovered at a mushroom farm, while the other three were later found at a nearby trucking business. An eighth victim is being treated in the hospital and is in critical condition.

Several children who had recently been let out of school and lived on the rural property saw the attack take place, San Mateo County Sheriff Christina Corpus said.

"This kind of shooting is horrific. It's a tragedy we hear about far too often, but today it's hit home here in San Mateo County," the office said.

"For children to witness this is unspeakable."

After carrying out the killings, the suspect drove to a local police station, where his arrest was caught on camera.

US news channels showed Zhao Chunli being pinned to the ground and arrested by police.

He was found with a semi-automatic pistol that may have been used in the attack and was cooperating with police, Sheriff Corpus said. 

San Mateo County Board of Supervisors President Dave Pine told the Associated Press news agency that a "disgruntled worker committed the attacks ".