A man is missing after a suspected fatal shark attack at a popular beach in Perth.

Emergency services raced to Sorrento Beach around 10.15am on Friday after reports of the incident, with the shark siren activated and the beach closed as a police boat and helicopter scoured the water.

Witnesses said the man, who has not been seen since, had been swimming around 50 metres offshore when the attack occurred.

Jacqui Rapaic, who was walking on the beach with a friend, is one of a number of people who reported seeing blood in the water.

“My friend said, ‘Jack, I think there’s been a shark attack’ and I looked out to the ocean and I could see a pool of blood,” she told the ABC.

“I hugged onto my friend… [we] felt rather traumatised at that point. It happened so quickly.”

A nurse who witnessed the incident told media she saw the swimmer being “mauled” by a shark.

“I only live just down the road, and I swim every day. I mean, there’s no way I’m going to be getting in that water again.”

Police have said they will officially confirm further details once they have established the circumstances of the incident.

The West Australian reported that some bystanders said the man was on a boogie board with a friend when the incident occurred, with the other man apparently uninjured.

A tradesman working on a nearby construction site told the newspaper he also saw a large “pool of blood” close to shore.

“When I saw that I already knew what was happening and then a bunch of people started crowding around and helicopters started showing up,” Daniel Saligari said.

It comes after a man was rushed to hospital on Monday following a shark attack on a remote WA beach.

Emergency services responded to Glenfield Beach, north of Geraldton in the state”s Mid West, shortly after 9.45am on Monday.

The man, who is in his 50s, was rushed to nearby Geraldton Health Campus under lights and sirens.

The surfer was seriously injured but is now in a stable condition.

Surf Lifesaving WA’s helicopter shark patrols began for summer last week.

–with AAP