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Daredevils were filmed launching themselves into the sea from 20ft-high sea walls in Portsmouth as other young people were seen swimming across shipping lanes.

Police warned the thrill-seekers they were risking their lives by 'tombstoning' into shallow water but were dubbed the 'fun police'.

Some took to the diving fad - which involves diving from height, sometimes while flipping in the air - during the recent heatwaves.

They jumped from the sea walls as well as shipping structures used by ferry company Wightlink, which links the Isle of Wight to the British mainland.

Others were spotted playing a game of 'chicken with a ferry' as they bathed in shipping lanes.

Near Portsmouth's historic docks, people swam across the Camber, directly in the path of the Isle of Wight ferries, as part of the craze.

Officers have increased seafront patrols as part of 'Operation Nautical' to discourage the activities, with Hampshire and Isle of Wight Police and Crime Commissioner Donna Jones calling tombstoning 'dangerous, irresponsible and simply not worth risking your life for'.

She added: 'Over the last four years I've invested in youth diversion programmes specifically in Old Portsmouth and along the seafront to protect young people and help improve the lives of residents.

Daredevils were filmed hurling themselves off 20ft-high sea walls in Portsmouth as part of a 'tombstoning' craze

'Do parents and carers know where their children are and what they're getting up to during the school holidays?

'They need to be having serious conversations with them. Keeping people safe takes more than just the police alone.'

Teenagers cheered adults hurling themselves in the sea as beachgoers watched on.

It is understood one man injured his neck while jumping off The Round Tower, a three-storey castle tower, after climbing over a safety rail.

Meanwhile boys in the sea shouted at police in a patrol boat, with one asking: 'Are you the fun police?'

Chief inspector Sam Warne said large crowds had gathered during the heatwaves and said high-visibility patrols would continue.

He also said Hampshire Police would consider dispersal orders and community resolutions.

Some 17 people were asked to leave designated areas between June 20 and 22 under a previous order - failing to comply could mean being arrested.

Mr Warne said it was 'incredibly important' that parents talk to their children about behaviour near the sea.

Police said the thrill-seekers were risking their lives by jumping into shallow water

'We are continually receiving reports concerning young people entering the water around The Camber area and diving into the water from Wightlink property,' Mr Warne added.

'I would remind people to stay away from restricted areas and ferry infrastructure and think about how your behaviour affects other people enjoying the seafront.'

The RNLI told those thinking of tombstoning to think about the stunt, branding it 'an incredibly high-risk activity'.

Sydney Horsted, lead lifeguard for West Sussex, said: 'The water depth changes with the tide - even if it looks deep enough, it may well be too shallow. There is also the risk of submerged objects under the water that you can't see until it's too late.

'In Southsea in particular, we have very strong lateral currents that can quickly sweep swimmers underneath the pier.

'The pier itself is also a very old structure, formed of sharp metal poles and wooden pylons that can cause cuts quite easily. These may also be submerged under the water.

'Even in the summer the water is cool and jumping into the sea on a hot sunny day can cause cold water shock, causing someone to panic when they enter the water suddenly.

'In this situation you should float on your back until you are calm, stretching out your arms and legs, tilting your head back and calming your breathing - you can then shout for help.'

A spokesman said The Round Tower in particular was dangerous to jump from because the only way back to shore was over rocks.

They added anyone seeing a person in danger in the water should call 999 and ask for the coastguard.