Thomas Williams, one of several witnesses who pulled the woman ashore, told the Orange County Register: "All of the back of her leg was kind of missing."
Williams said the woman was "not calm" but was conscious, and that a rubber surfboard leash was used as a tourniquet to stop the bleeding.
According to the Guardian, it's thought the attack was made by a great white shark or a seven-gill shark.
The Telegraph reports that the beach is adjacent to the Camp Pendleton Marine Base. Marine Sgt Asia Sorenson said the victim - a civilian - was airlifted to a hospital in unknown condition.
The paper added that last year, a woman was bitten by a shark while swimming off Corona del Mar, about 25 miles to the north of the Saturday attack.
Experts said at the time that the the bite marks on her chest, hip and shoulder suggested that the shark was at least 10ft in length.
World’s Worst Shark Attacks
World’s Worst Shark Attacks
Back in 1959, Robert Pamperin was diving for sea snails in La Jolla Cove, California when he was attacked by a shark. His diving companion Gerald Lehrer painted a vivid picture of the events that day. According to reports, Gerald turned to see his friend rising unusually high in the water. He dove below the surface and saw his friend waist-deep inside the shark’s jaws. Despite his efforts to distract the shark, Robert was dragged to the sea bed in the jaws of the 22-foot shark.
Professional surfer Bethany Hamilton has proven that not even a shark attack can keep a girl down.
At the age of 13, while surfing off Kauai’s north shore in Hawaii, Hamilton was attacked by a 14-foot tiger shark and was left with a severed left arm.
After losing more than 60% of her blood, Hamilton required several surgeries, but she did not let the incident impact her dreams of surfing.
Miraculously, just one month after the attack, Hamilton returned to the water to continue pursuing her goal to become a professional surfer. Shortly thereafter, she made her return to surf competition; placing 5th in the Open Women’s division of that contest. With no intention of stopping, Bethany continued to enter and excel in competition.
Just over a year after the attack she took 1st place in the Explorer Women’s division of the 2005 NSSA National Championships – winning her first National Title.
In 2010 she was the 20th ranked woman among surfers and her autobiography Soul Surfer: A True Story of Faith, Family, and Fighting to Get Back was made into a movie with Dennis Quaid and Carrie Underwood in 2011.
When HMS Birkenhead struck a rock just three miles from the coastline in South Africa, the disaster was only just beginning for its crew.
After the captain commanded that ‘women and children’ take the undamaged lifeboats, the rest of the 643-strong crew made up of British and Irish soldiers, were left on board to go down with their ship.
Tragically, once the vessel sank, the men were not given the opportunity to swim to safety. Sharks surrounded the floating men and embarked on a feeding frenzy.
According to one report, in a very short time many of those who survived the sinking were dead. And on the surface of the water, which was covered in blood, floated the almost unrecognisable remains of those who had been attacked by sharks.
Just 193 people survived the disaster.
Rodney Fox was just 24 years old when he was viciously attacked by a great white shark, while competing in a spearfishing competition off the south Australian coast. It later turned out that they only thing keeping Rodney’s internal organs from falling out of his body -- and him alive -- was his wet suit.
After a nightmarish shark attack that left his ribs broken, one lung ripped open, the main artery from his heart exposed, diaphragm punctured and right arm flayed to the bone, Fox needed 462 stitches to put him back together.
Reports say that Rodney went on to design and build the first underwater observation cage to dive with the great white shark, and for over 40 years has led major expeditions to film and study his attacker.
The coast of New Jersey was struck by a series of shark attacks back in 1916. During a heat wave that saw Americans flock to the beach, four people were the victim of shark attacks.
According to The Telegraph, the first victim was 25-year-old Charles Vansant, who bled to death after sharks stripped the flesh off his thigh as he went for an early-evening swim.
Less than a week later, a 27-year-man suffered severe injuries to his stomach, and severed legs, while swimming at a beach just 45 miles from the first attack.
Six days later, further north up the coast, 12-year-old Lester Stillwell was dragged underwater as he played in the sea with his friends.
Tragically, attempts by his father to save his son, resulted in his also being attacked and bleeding to death.
When British naval ship HMS Valerian capsized during a hurricane, near the coast of Bermuda, the surviving crew members were savaged by sharks in the water.
There were 88 fatalities, most of them from shark attacks, according to reports.
As the sailors hung on the life rafts for dear life, sharks pulled some of them off and into the water, where they were bitten and chewed in a shark feeding frenzy, reveals sharkfacts.com.
Rescue boats managed to pull just 20 survivors from the waters.
Perhaps the most hard-to-read shark attack story of recent years was that of Ian Redmond, 30, who was attacked and killed in the Seychelles while on his honeymoon.
In 2011, Redmond went snorkelling 20 yards from the shore, where his new wife was sunbathing, and was attacked by a six-foot shark.
An American tourist told the Daily Mail that someone saw a fin sticking out above the water. According to the tourist, a woman then ran over, screaming, "That’s my husband! We were just married."
Redmond wasn’t the first man to be attacked off Anse Lazio beach, Paslin island. Just two weeks before, a 36-year-old French tourist was killed by a shark off the same beach.
In 1945, when two Japanese torpedoes hit the USS Indianapolis in succession, a chain reaction of explosions ripped the ship in two and of the 1,196 individuals aboard, just 900 made it into the water alive.
However, that moment was just the beginning of one of the worst shark attacks in history.
For four days the crew bobbed in the water while the sharks feed off their group, picking off the dead and injured first.
By the time a rescue crew arrived only 317 individuals remained alive.
According to Salon.com, estimates of the number who died from shark attacks range from a few dozen to almost 150.