At least one gunman opened fire Thursday at a popular San Francisco park packed with families and tourists, leaving three people wounded and sending dozens of panicked people running for cover, witnesses and police said.

Police are looking for at least one gunman who fled the scene at Dolores Park, San Francisco Police Officer Grace Gatpandan said.

San Francisco General Hospital spokesman Brent Andrew said one of the victims remained in critical condition Thursday evening.

Another man was treated and released, he said. The third victim, also a male, is a minor and remains hospitalized but Andrew wouldn't provide any other details about him.

A man who witnessed a shooting said it happened after about half a dozen men who covered all but their eyes started shouting and acting threateningly on a park's bridge.

The man, who identified himself to the San Francisco Chronicle, only as Hatter said several park regulars confronted the group and that he approached them to make sure everything was OK.

As he got close someone shouted that the group had guns and that a second later one of the men pulled out a handgun and fired at least six rounds, Hatter said.

Two people directly behind him were struck by bullets. He escaped with only an injury, possibly from "shrapnel," to his leg, he said.

"I normally can handle (myself) well in these situations, but I was pretty sure I had just lost my life," Hatter said.

Nearby resident Antonia Juhasz said she was sitting in Dolores Park when she heard a burst of gunshots.

"I saw a person with a gun in their hand running," she said. "I think there were a total of three shots."

Juhasz, 47, said she saw two people who had been shot. Both were bleeding as emergency workers carried them away on stretchers, she said.

San Francisco police advised people to stay away from Dolores Park right after the shooting but lifted that advisory two hours later.

Dolores Park sits on a hill in the Mission District and is a popular destination for locals and tourists who come to sunbathe and take in city views. It's near a high school.

Juhasz, who has lived in the neighborhood for more than 20 years, said the park was packed with families and tourists when the shots were fired.

"At first people didn't totally react because it sounded like fireworks," said Juhasz, a writer and freelance journalist. "I was yelling at people, 'It's actually a gun, it's actually a gun.'"

"It was terrifying, mostly because people weren't reacting," she added.

People began running after realizing there had been gunshots, she said.