A teenage girl who apparently jumped into a river to swim with dolphins has been fatally mauled in a rare shark attack in a Western Australian river.

Police said the 16-year-old’s friends witnessed the horrifying tragedy unfold in the Swan River in North Fremantle on Saturday afternoon.

Onlookers also described terrifying scenes as swimmers rushed out of the water screaming.

It was the first fatal shark attack in the river in 100 years and considered “unusual” for sharks to be that far down.

Police scour the Swan River after the shocking tragedy. Photo: 7NEWS screenshot

Police said the young girl had been riding jet skis with friends when she spotted a pod of dolphins near the old Fremantle traffic bridge around 3.20pm.

“They were on jet skis. There was possibly a pod of dolphins seen nearby and the young female jumped in the water to swim nearby the dolphins,” said WA Police Acting Inspector Paul Robinson.

“The family weren’t there when this took place however, her friends were and as you can imagine this is an extremely traumatic incident for anyone to witness so obviously we’re offering counselling services to anyone who did witness it or is affected by the incident.

“It’s an extremely traumatic event for everyone involved and everyone who knew the young girl, so I won’t be going into the extent of the injuries,” he said.

Acting Inspector Robinson said it was “unusual for a shark to be that far down the river”.

“I’ve spoken to fisheries and water police and unfortunately we have no idea what type of shark it is,” he said.

Acting Inspector Robinson said the Perth girl was pulled from the water, with media reporting a man was being hailed a ‘hero’ for jumping into the water to try and save her.

Witnesses described the victim’s injuries which were so horrific that emergency services who rushed to the scene were unable to revive her.

Media said this man had jumped into the water to try and save the 16-year-old girl. Photo: 7NEWS screenshot

The shocking incident was the first fatal shark attack in almost 100 years.

On January 31, 1923, a 13-year-old student, Charles Robinson, was bitten on the leg while swimming near his school’s boat shed at Freshwater Bay.

The most recent shark-related incident happened on January 2021 when a swimmer, Cameron Wrathall was attacked near Blackwall Reach by a suspected 2-3 metre bull shark.

Police have asked anyone who witnessed the event to contact them.

Fremantle detectives will prepare a report for the coroner.