A swimmer has been eaten by a four-metre-long great white shark in front of terrified beachgoers who recall hearing the man's ear-piercing screams as blood was seen rising from the water.

Emergency services rushed to Buchan Point near Little Bay Beach in Sydney's east just after 4.30pm on Wednesday following reports of a shark attack.

Horrifying video from the scene shows local fishermen watching on helplessly as the swimmer is attacked by the monster shark, which they saw 'swallow parts of his body' after ripping it in two.

After the alarm was raised, rescue helicopters and lifesavers on jet skis were sent out in search of the swimmer and the killer predator, with dozens of local swimmers ordered out of the water.