San Diego lifeguards ordered a stretch of beach in La Jolla closed Saturday afternoon after a confirmed sighting of an 8- to 10-foot hammerhead shark.
The closure, ordered at 1 p.m., will remain in place for at least 24 hours from La Jolla Cove to Scripps Pier, authorities said.
Swimmers and others who had come to the popular beach seeking relief from the oppressive heat quickly scrambled out of the water as lifeguards warned of possible danger.
No one was injured but the shark seemed to be acting aggressively toward a group of kayakers, authorities said.
"The shark reportedly followed the kayakers all the way to the beach and was last seen outside the surf line at the foot of Avenida de la Playa at the south end of La Jolla Shores," said Lee Swanson, spokesman for the San Diego Fire and Rescue Department.
Lifeguards in a boat were searching for the shark. A helicopter from the Fire and Rescue Department joined the search.
The presence of the shark was first reported by the kayakers. After seeing video of the shark, lifeguards immediately ordered the beach closed.
"There have been no additional sightings and no other reports since the original report," Swanson said.
Lifeguards contacted Andy Nosal, marine biologist at Scripps Institution of Oceanography, who confirmed that "the size, species and behavior of the shark warranted the closure," Swanson said.