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A boy suffered a suspected broken ankle and knee after leaping into the sea from Broadstairs Pier.
Coastguard and ambulance crews were called after the youngster injured himself as he hit the seabed during the the reckless jump.
He managed to make his way to shore before emergency workers arrived, and was given first aid by an onlooker.
HM Coastguard Margate said: "The person was given painkillers, was put on our stretcher and officers assisted in evacuating him from the beach."
The incident on Friday has since sparked a warning against jumping or diving from piers or groynes into the water - a practice commonly referred to as tombstoning.
The coastguard says it is often impossible to judge the depth of the water or see hidden hazards such as rocks.
There have been 20 tombstoning deaths in the UK since 2005 and 70 reported injuries.