A gunman opened fire in a southern California church during a lunch banquet yesterday, killing one person and wounding five, before churchgoers detained the suspect and tied his legs with an electrical cord, authorities have said.

Police responded to the incident that unfolded at about 1.30pm local time (9.30pm Irish time) at the Geneva Presbyterian Church in Laguna Woods and arrested a man in his 60s, Orange County Undersheriff Jeff Hallock told a news conference.

"That group of churchgoers displayed ... exceptional heroism and bravery in intervening to stop the suspect. They undoubtedly prevented additional injuries and fatalities," he said.

One person died at the scene while four other victims were critically wounded. Another person suffered minor injuries, he said. All of the injured were taken to hospitals.

About 30 to 40 people were in the church when the shooting occurred, Mr Hallock said.

"At this time, we do not know what the suspect's motive maybe or whether he had an intended target, or whether this is even a hate-related incident," he said, adding that authorities believe the suspect does not live in the area.

Two handguns were found at the scene.

"No one should have to fear going to their place of worship. Our thoughts are with the victims, community, and all those impacted by this tragic event," Governor Gavin Newsom's office said on Twitter.

The victims were all of Taiwanese descent, Taipei's foreign ministry said, adding that it had "asked our representative office to express our most sincere and deepest condolences to those killed or injured in the shooting and their families."

According to the sheriff's department, the victims were aged between 66 and 92.

Investigators were still searching for a motive, Mr Hallock said, adding that the alleged gunman, who was not injured during the incident, was believed to be an Asian male in his 60s.

"The man is not believed to live in the area and investigators are working to determine his city of residence and whether he has any connections to the church or its congregants," the sheriff's department said in a statement.

It was at least the second mass shooting of the weekend int he United States, which has been plagued with gun violence in recent years.

In Buffalo, New York, on Saturday, a white 18-year-old man opened fire at a supermarket in a mostly Black neighborhood, killing ten and wounding three in what authorities described as a purely racist attack.