The shark was spotted about 150 to 200 yards offshore at about 1 p.m. near Linda Mar Beach, a section of Pacifica State Beach, Pacifica police Capt. Fernando Realyvasquez said.
Surfers, boaters and people on the sand reported that the shark was 18 feet long or larger.
"It was pretty convincing based on the half-dozen witnesses that reported seeing it from different vantage points," Realyvasquez said.
A surfer who was in the water at the time of the attack said he saw a large shark thrashing around with a sea lion in its mouth and a large amount of blood in the water.
Witnesses on the beach said that after the attack, two men in a small boat drove up and down the shoreline telling surfers and swimmers to get out of the water.
Signs warning beachgoers of the incident were posted today, Realyvasquez said, the first time this year a sighting has led to such a precaution in Pacifica.