Three people were killed and scores more wounded after a gunman opened fire at a food festival in California on Sunday.

The suspected gunman was killed but the chief of police in Gilroy, a city about 50 kilometres south-east of San Jose, gave a warning that he may have an accomplice.

"It's just incredibly sad and disheartening that at an event that does so much good the community had to suffer from a tragedy like this," Police Chief Scott Smithee said at the site of the city's garlic festival.

Mr Smithee said two people used a tool to cut through a fence and then one of them opened fire.

Footage on NBC showed people running as shots rang out.

A witness named Julissa Contreras said a white man in his 30s armed with a rifle opened fire indiscriminately.

"I could see him shooting in just every direction. He wasn't aiming at anyone specifically. It was just left to right, right to left," Ms Contreras said. "He definitely was prepared for what he was doing."

City councilman Dion Bracco said at least three people had died according to various US media outlets.

Senseless violence
California Governor Gavin Newsom said the incident was "nothing short of horrific".

"Tonight, [California] stands with the Gilroy community. My office is monitoring the situation closely," he tweeted.

"Grateful for the law enforcement's efforts and their continued work as this situation develops."

The Mercury News reported that the shooting took place as the festival was winding down.

Stage hand Shawn Viaggi hit the ground after hearing "loud pops," it reported.

"I called out, 'It's a real gun, let's get out of here,' and we hid under the stage," Viaggi said.

Thirteen-year-old Evenny Reyes told the paper that "we were just leaving and we saw a guy with a bandanna wrapped around his leg because he got shot. And there were people on the ground, crying."

Police stand guard outside Gilroy High School following a shooting.San Jose Mercury News via AP
Police stand guard outside Gilroy High School following a shooting.San Jose Mercury News via AP
"There was a little kid hurt on the ground. People were throwing tables and cutting fences to get out," said Reyes.

"The hearts of Gilroy PD and entire community go out to the victims of today's shooting at the Garlic Festival," the local police department tweeted.

"Grateful to first responders who are on the scene in Gilroy and keeping those injured by such senseless violence in my thoughts," Senator Kamala Harris, who is running for the 2020 Democratic presidential nomination, wrote on Twitter.

San Jose mayor Sam Liccardo tweeted: "Praying for lives and recoveries of each of the shooting victims at #GilroyGarlicFestival. We will continue to support our neighbors in any way we can."

Shootings are a frequent occurrence in the United States, but despite the scale of the gun violence problem in the country, efforts to address