With an hour of daylight remaining, the search at Port Beach in North Fremantle will likely be called off to be continued at “first light” tomorrow.
Witnesses believe the shark was a 4.5-metre great white.
A short time before the attack, a large shark was seen in the area.
The attack was witnessed by a group of teenage boys around 10am who tried to help the victim and raised the alarm.
However when the shark attacked a second time, they raced their boat, a metal dinghy, along the shoreline to clear the water of other swimmers and alert authorities.
St Johns Ambulance crews were on scene at Port Beach in Fremantle just after 10am local time.
“We looked over and a kid was coming, flying down, trying to tell everyone to get out of the water,” one witness told 9News.
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.
The attack has forced the closure of several beaches along the coastline.
Port Beach was quickly closed, followed by Leighton Beach and Cottesloe Beach.