Swimmer missing after shark attack at Perth beach

Police are searching for a man in his 50s after a shark attack off a popular Western Australian beach.

St Johns Ambulance crews attended Port Beach in Fremantle at 10am local time.

The attack was witnessed by a group of teenage boys who tried to help the victim, then warned other swimmers to get out of the water.

"We looked over and a kid was coming, flying down, trying to tell everyone to get out of the water," one witness told 9News.

Witnesses believe the shark was a 4.5-metre great white.

Western Australia police said the search will continue into the night and will resume at "first light".

Acting Inspector Troy Douglas praised the actions of the boys who alerted emergency services to the attack "pretty much straight away".

The victim was a regular swimmer at Port Beach, police said.

Police boats and lifesavers are searching for the victim in the water. His condition is not known.

Police boats and other vessels are in the water.

Several beaches along the coast has been closed following the attack.

The attack has forced the closure of several beaches along the coastline.

Port Beach was quickly closed, followed by Leighton Beach and Cottesloe Beach.

Surf Life Saving reported an unknown species of shark about two metres long had been spotted off Port Beach just after 10am.