NEWPORT BEACH, Calif. » Thousands of Memorial Day beachgoers were kept out of the water today as lifeguards searched miles of popular Southern California shoreline for a shark they believe attacked a swimmer the day before.

The woman, who was swimming in a wetsuit, received large bite marks on her upper torso and shoulder and was bleeding heavily after a lifeguard boat spotted her in distress Sunday at Corona Del Mar State Beach, said Tara Finnigan, spokeswoman for the city of Newport Beach.

The woman’s condition was not immediately available, but she was conscious when she went to a hospital, Finnegan said.

“We do suspect the victim was bitten by a shark, but haven’t been able to confirm that because we had no other witnesses and there was no reported shark sighting before the incident or after,” Rob Williams, chief lifeguard of the Newport Beach Fire Department’s Marine Operations Division said in a statement.

Lifeguards were searching the water again Monday, with police in a helicopter assisting, Williams said.

“We are treating this as a shark-bite incident and are asking everyone to please stay out of the water in the closure area,” he said.

While people were welcome on the beachfront and pier, lifeguards were asking anybody entering the water to return to the shore. With skies overcast and temperatures chilly Monday, several beachgoers said it was too cold to swim anyway.

The ban on entering the water stretched for several miles from the Balboa Pier to the city limits at Crystal Cove State Beach. Officials said it would remain in effect until further notice.

The injured woman was swimming about 100 yards offshore, just outside buoys marking a protected swimming area near where boats travel.

Lifeguards who saw her struggling witnessed the last part of the attack and quickly ruled out any kind of boating or watercraft accident, Williams said.

There have been no recent reports of sharks in the area, he said.