A 12-year-old boy is fighting for his life after a shark attacked him at a popular Sydney beach, leaving him with critical injuries to both legs.

Five of the victim’s friends watched in horror as the attack unfolded at Nielsen Park in Vaucluse before desperately trying to help until emergency services arrived.

The attack happened at about 4.30pm at the beach, which has both a netted swimming area and open water.

Police responded with “multiple resources”, including intensive care paramedics and aeromedical teams deployed by helicopter.

Up to five other children were with the boy at the time of the attack and attempted to assist him before emergency crews reached the scene.

He was transported by boat to Rose Bay for urgent treatment, while officers comforted shocked children at the beach.

New South Wales police said Water Police officers “attached two medical tourniquets” before administering first aid aboard the Water Police 41 vessel.

The boy had sustained “serious leg injuries” and received further medical attention after arriving at hospital, police added.

“[His] injuries are consistent with what is believed to have been a large shark,” New South Wales police said.

An ambulance spokesperson said: “The patient, a teenage boy, with critical injuries to both of his legs, was transported to Sydney Children’s Hospital, Randwick, in a critical condition.”

It is understood the boy was jumping off rocks at one end of the beach when the shark appeared.

Police have closed the beach and urged swimmers to avoid the area.

Officers are still searching for the shark responsible and have cordoned off the rock ledge where the children were jumping.

The boy was pulled from the water “within minutes” of witnesses alerting police, the force said.

Agriculture Minister Tara Moriarty said the Department of Primary Industries and Regional Development is working to identify the species of shark involved in the attack.

Ms Moriarty said: “This is a tragic shark attack on a young boy having a swim on a Sunday afternoon in a harbour beach in Sydney’s east.

“Our thoughts are with the young boy and his family.”

She added: “I understand there were also a lot of young people on the beach at the time of the attack. Our thoughts are also with them.”