A woman was sexually assaulted while her male companion was forced to lay face down in the sand at a Southern California beach just after midnight Sunday, police said.

The attacker, who is still at large, "simulated the use of a handgun" after sneaking up behind the victims, who were sitting in the sand in the 3400 block of the waterline at 12:16 a.m., according to a press release from the Manhattan Beach Police Department.

The attacker also robbed the man and woman, in their 40s, before the attacker fled on foot, police said. He was last seen walking away from the beach and heading south.

Police searched the area for the man, but couldn't find him. Detectives were following up with the investigation.

"We've never had a crime at all like this, certainly not on the beach," said Manhattan Beach Mayor Wayne Powell, who lives two blocks from where the crime took place. "When I found out about it, it was terrifying."

The tight-knit beach community was in disbelief as to what happened to the couple sitting in the sand.

"It's just unbelievable to me that that could happen here," resident Jane Sager said.

Manhattan Beach police did not have a suspect description available as of Sunday night.

Powell said detectives combed the beach for evidence and were looking for possible surveillance video to help catch the attacker.

"We're going to find you and put you behind bars,” the mayor said.