A gunman motivated by hatred against Taiwan chained shut the doors of a California church and hid firebombs before shooting at a gathering of mostly elderly Taiwanese parishioners, killing a man who tackled him, US authorities said.

David Chou, 68, of Las Vegas, a US citizen whom authorities say grew up in Taiwan, drove to Orange County on Saturday and the next day attended a lunch held by Irvine Taiwanese Presbyterian Church, which worships at Geneva Presbyterian Church in the community of Laguna Woods.

Though he knew no one there, he spent about an hour mingling with about 40 attendees and then executed his plot, authorities said at a news conference.

He chained the doors and put super glue in the keyholes. He had two 9mm handguns - legally purchased years ago in Las Vegas - and three bags, containing among other things four Molotov-cocktail-type incendiary devices and extra ammunition.

He opened fire and in the ensuing chaos Dr John Cheng, 52, tackled him, allowing other parishioners to subdue him and tie him up with extension cords.

Cheng died and five people were wounded, the oldest 92. Sheriff Don Barnes called Cheng's heroism "a meeting of good versus evil" that probably saved the lives "of upwards of dozens of people".

Chou was booked on suspicion of murder and attempted murder and jailed on $1 million bail. He was expected to appear in state court on Tuesday. It was not immediately clear whether he had an attorney who could speak on his behalf. A federal hate crimes investigation is also ongoing.

Chou had worked as a security guard in Las Vegas, authorities said.

There was no immediate word on why Chou chose to target the church in Laguna Woods, a scenic coastal area whose population mainly consists of retirees and is near a large gated community.

Barnes said the motive for the shooting was Chou's hatred toward Taiwan that was documented in handwritten notes that authorities found. Chou's family apparently was among many forcibly removed from mainland China to Taiwan sometime after 1948, Orange County District Attorney Todd Spitzer said.

Barnes referred to Chou as an immigrant from China but Taiwan's Central News Agency says it interviewed Louis M. Huang, director general of the Taipei Economic and Cultural Office in Los Angeles, and he confirmed that Chou was born in Taiwan in 1953.

Barnes said Chou acted alone and was "not believed to be associated with any specific church or any religion, and there's no direct connection to the church or any member of the church that we're aware of".

Balmore Orellana, a former neighbor, said Chou's life unravelled after his wife left him last year. Before, Chou had been a pleasant man who used to own the Las Vegas apartment building where he lived until being evicted in February, Orellana told the Associated Press.

Chinese embassy spokesperson Liu Pengyu told the AP via email that the Chinese government has "consistently condemned incidents of violence. We express our condolences to the victims and sincere sympathy to the bereaved families and the injured".

Those wounded by gunshots included an 86-year-old woman as well as four men, ages 66, 75, 82 and 92, the sheriff's department said. Authorities on Monday said two of the wounded were in good condition, two were in stable condition and the status of the fifth patient was undetermined.

Jerry Chen, a long-time member of the church, said a group of about 40 congregants had gathered in the fellowship hall for a luncheon after a morning service to welcome their former Pastor Billy Chang, a beloved and respected community member who had served the church for 20 years.

Everyone had just finished lunch and were taking photos with Chang when Chen went into the kitchen. That's when he heard the gunshots.

Barnes said Cheng, a sports medicine doctor who is survived by a wife and two children, charged at the shooter and attempted to disarm him, allowing others to intervene. Chang hit the gunman on the head with a chair before other parishioners subdued him.

"I will tell you that evil was in that church," Spitzer said, who added that Chou had "an absolute bias" against Taiwan and its people.

The shooting came a day after an 18-year-old man shot and killed 10 people at a supermarket in Buffalo, New York, in a racist rampage where the white gunman allegedly targeted a supermarket in a predominantly Black neighbourhood.