The incident happened around 9:30 a.m. when a fisherman standing on the pier had a 7-to 8-foot long juvenile great white shark hooked on his line. He spent 30 to 40 minutes trying to reel it in.
As the shark struggled, a group of long-distance swimmers came by. That's when the shark bit the victim in the upper-right torso, Rick Flores with L.A. County Fire said.
Steven Robles, a 50-year-old real estate broker from Lomita, says he was doing a two-mile routine swim that he does every weekend with a group of 14 friends when the shark suddenly appeared out of nowhere.
"It came from the bottom of the water. It came up to the surface, it looked at me and attacked me right on the side of my chest," Robles said. "That all happened within two seconds, I saw the eyes of the shark as I was seeing it swim towards me. It lunged at my chest, and it locked into my chest."
Robles says he grabbed the shark by the nose to try to pry its mouth open, but the shark disappeared as quickly as it came.
"Instinctively, I took my hand and grabbed its nose and tried to pry it off my chest, and, in so doing that he released himself and it just swam away," Robles said.
Fellow swimmers and paddle boarders rushed to his aid to get him back to shore, with the help of L.A. County lifeguards.
Jeff Garnevicus was surfing near the pier, heard his screams, and paddled over to help.
"As everybody tried to get him on the paddle board, that's when we noticed the big gouge out of his side," Garnevicus said.
Paramedics treated Robles on-scene. Amazingly, the sharks sharp teeth and powerful jaws did not puncture any organs or major arteries. Robles was transferred to Harbor UCLA Medical Center with moderate wounds, and is now recovering at home.
"I just got really lucky," Robles said. "It could have easily gone the other way."
The shark swam off and disappeared into the murky water. Helicopters and lifeguards searched for the shark, prompting a stretch of water from Hermosa Beach to the Manhattan Beach pier to temporarily close. It reopened around 2 p.m.
Great whites are common in the waters off Manhattan Beach.
Eric Martin, who works at the Roundhouse Aquarium on the pier, says that this may have been what's known as a response bite.
"It's where the shark is hooked in the lip, and he's trying to shake the hook," Martin said. "His mouth is opening and closing, opening and closing, shaking his head, and when he's doing that, of course, his teeth are coming out and we think the swimmer just happened to be at the wrong place at the wrong time."
It's not clear if the fisherman involved in this incident was chumming for sharks, which is illegal. Authorities say the fisherman hasn't been sited, but the investigation is still ongoing.
Fishing is prohibited from the Manhattan Beach Pier until Tuesday. All other legal activity is allowed, according to Manhattan Beach police.
ABC News contributed to this report.