A single mother-of-three is fighting for life after she was attacked by a shark.

Leeanne Ericson, 35, was swimming at a popular surf spot off the coast of San Diego, California on Saturday when she was attacked by a shark.

The single mum of three young children is now fighting for life in hospital, her mother said.

On a GoFundMe page set up to help with Leeanne's recovery, her mother Christine McKnerney Leidle said the shark "tore her right back leg from her glute to her knee".

According to the Los Angeles Times, Leeanne was pulled to the shore by members of the public, who helped control her bleeding until she could be airlifted to hospital.

Thomas Williams was one of the strangers who jumped to help Leeanne after the attack.

"It was definitely to the point, her hamstring was gone," the 29-year-old, who had just passed his EMT training test told the Orange County Register.

"If she didn’t receive immediate care, it was life-threatening. All of the back of her leg was kind of missing."

The beach was immediately closed after Leeanne was attacked around 6:30pm, with California State Parks officials and Marine Corps commanders looking to extend the closure.

"We've closed the beaches in the geographic proximity to the attack," Todd Lewis, the Central Sector superintendent for California State Parks' Orange Coast District told the Los Angeles Times.

Members of the pubic are banned from swimming, surfing or diving a mile north and south of the beach where the attack occurred.

Leeanne's mother wrote that doctors says "she drowned when the shark pulled her under" and is now facing multiple surgeries before she can recover.

"She is a single mum with three young children who depend on her," she wrote.

"I don't know how much this will cost but she's going to need all the help she can get."