A large great white shark has forced swimmers from the water during WA's Rottnest Channel Swim.

About 100 people were pulled from the water on Saturday after a competitor saw the predator, estimated at three to four metres long, swim beneath him.

Organisers enacted a risk management plan and evacuated anyone within a 1km radius.

Helicopters flew above after the sighting to monitor the situation.

A competitor saw the predator, estimated at three to four metres long, swim beneath him. (Getty)

More than 2500 people participated in the annual event, with ex-Premier Colin Barnett sounding the starting horn.

A support boat was towed after it began to sink about 8km in, with five people pulled on to a rescue vessel, Fremantle Sea Rescue tweeted.

Solomon Wright took out the solo men's title and set a new record of 3:59:28, shaving 47 seconds off the previous time from 2000.

"I'm happy with the win, the record is just the cherry on top," he told Perth Now.

Rescue vessels have attended a 9m boat that started sinking approximately 8km into the Rottnest Channel Swim. (Twitter/@FreoSeaRescue)

Olympic swimmer Heidi Gan was the fastest solo female swimmer, with a time of 4:21:55.

A total of 220 did not finish the race, including those pulled from the water.

The event is a 19.7km open water swim from Cottesloe Beach to Rottnest Island.