A six-year-old boy was among three people killed after a gunman opened fire at a food festival in California.

Steven Romero had been visiting the garlic festival in the city of Gilroy with his mother and grandmother when he was shot and killed, according to NBC Bay Area.

Another 15 people were injured in the attack at 5.41pm local time (1.41am Irish time) after the gunman began "shooting in every direction".

He killed three people before being shot dead by police in attendance within a minute.

Steven's father, Alberto, said he was at home when he got the call that his son had been killed.

"My son had his whole life to live and he was only six," he told NBC Bay Area.

When asked what his thoughts were following the tragedy, he said: "Hopefully they get the shooters and basically that's it right now.

"I lost my son and that's the only thing I really can do besides trying to be with him until I could put him in his resting spot, wherever that is."

A young couple embrace at a parking lot after a shooting at the Gilroy Garlic Festival in Gilroy, California | Image: Thomas Mendoza/AP/Press Association Images

Chief of police Scot Smithee said the gunman, and a suspected second gunman or supporter, came into the festival via a creek which borders a parking area.

He said it was thought they used "some sort of a tool to cut through the fence".

Mr Smithee confirmed it was still an active scene.

"There were reports of a shooting on the north side of the garlic festival area," he said. "Officers were in that area and engaged that suspect in less than a minute. The suspect was shot and killed."

Witness Julissa Contreras told reporters she saw the gunman firing a rifle that was "able to shoot three to four shots a second".

"It was just rapid firing," she said.

"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. He definitely was prepared for what he was doing."

A number of videos posted to social media showed people running away from the event. Gunshots could be heard in the background of some of the clips.

In one of the videos a woman can be heard asking: "Who would shoot up a garlic festival?"

Julissa Contreras said she was about 20ft from the shooter when he opened fire.

In a video posted on social media, she said the alleged shooter was a white man wearing a "utility outfit, like he looked like he was private security".

The suspect shot at police officers with what appeared to be a "semi-automatic rifle".

A 13-year-old girl told San Jose Mercury News she had initially thought the noise had been fireworks, but saw a man that had a bandana tied around his leg wound.

The singer of a band who had been performing at the festival said he saw a man wearing a green shirt and a handkerchief around his neck firing into the food area with what appeared to be an assault rifle.

Jack van Breen and his bandmates dived under the stage before hearing someone shout: "Why are you doing this?"

He said the reply came "Because I'm really angry".

US President Donald Trump tweeted: "Law Enforcement is at the scene of shootings in Gilroy, California. Reports are that shooter has not yet been apprehended. Be careful and safe!"

Law Enforcement is at the scene of shootings in Gilroy, California. Reports are that shooter has not yet been apprehended. Be careful and safe!

— Donald J. Trump (@realDonaldTrump) July 29, 2019

The governor of California, Gavin Newsom tweeted: "This is nothing short of horrific. Tonight, CA stands with the Gilroy community.

"My office is monitoring the situation closely. Grateful for the law enforcement's efforts and their continued work as this situation develops."

This is nothing short of horrific. Tonight, CA stands with the Gilroy community. My office is monitoring the situation closely. Grateful for the law enforcement’s efforts and their continued work as this situation develops. https://t.co/GrtHjtCEz7

— Gavin Newsom (@GavinNewsom) July 29, 2019

One of the largest food festivals in the US, the Gilroy Garlic Festival is an annual three-day event south of San Jose at the outdoor Christmas Hill Park.

The event's website says weapons of any kind are prohibited and that 4,000 people work and volunteer at the festival.