STINSON BEACH, Calif. -- A suspected great white shark has been spotted in the waters off Stinson Beach, triggering the National Park Service Monday to restrict water access at the popular Labor Day destination.

Officials said the shark was spotted around 7 p.m. Sunday about 125 yards offshore, north of the beach's main lifeguard tower, by a surfer who worked as a lifeguard for the Park Service last year.

The shark was thought to be a great white because of its size and the shape of its tail, Park Service spokeswoman Ozola Cody said.

John Ralph, chief lifeguard at Stinson Beach, said the shark was described as 8 to 10 feet long, which may indicate it is young.

"This shark had a lot of girth," he said.

Officials posted signs Monday to warn visitors of the sighting, and beachgoers will not be able to go in water past their knees.

Park Service officials will monitor the waters through Friday.

Ralf said "sharks are always out there" but that the last shark sighting at the beach was in May 2007.

He said the Park Service has informed the Stinson Beach Fire Protection District and other local agencies of the sighting, and that local surf shops will also be contacted.