Coastguards have warned the public to stay away from rock armour on the Yorkshire coast after rescuing the teenager.
The 15-year-old was fishing for mackerel when his leg slipped between two large rocks which are part of the sea defences at Staithes, north of Whitby.
Fire crews had to use high pressure air bags to force the rocks apart so the lower half of his limb could be released.
A spokesman said: “At a different time, with the tide coming in, the consequences could have been fatal.”
The casualty, believed to be on holiday from Oxfordshire, was allowed home after a check-up by ambulance crews.
The incident happened on the eastern side of Staithes on Thursday evening. Firefighters were called out from Lythe, Whitby, and Scarborough and worked under floodlights to release the boy.
Scarborough Council wants to install boulders around the cliffs beneath the Spa, obliterating an area of sands called Children’s Corner since the 19th century.
Protesters have slammed the £16.6m scheme as a massive waste of taxpayers’ money and say the existing wall should be repaired and renovated instead.
Council bosses say the existing sea defences are not adequate to prevent massive landslips which will put hundreds of homes at risk.
Sons of Neptune leader Freddie Drabble said: “No one’s life should be put at risk in this way.
“It is lucky this has happened now when there is still a chance of stopping similar accidents happening in Scarborough.
“The risk has been highlighted by this accident which could have been a fatality and we need to go back to the drawing board.
“It is uncanny that this has come so soon after the prediction a lifeguard made at the public meeting in April – that there would be accidents.
“It is all about children being trapped in rat-infested rock armour. This is going to replace the sand for much of the time at Children’s Corner and will create a climbing attraction.
“Kids will kick their balls up there and pets will chase them and not be able to get out. It is an absolute nightmare.
“We are losing 10 and half thousands square metres of beach – about half of the sands – and creating a 350m-long barrage.
“These crevices between the boulders will cause injuries or worse and the council will be absolutely liable for creating a danger.
“It will be automatic compensation. Children cannot see the danger of this. They will climb on there and will be trapped as we have seen by this latest incident.
“I have written to the leader of the council and the Environment Agency calling on them to forget the whole idea, go back to consultation about reinforcing the existing sea wall otherwise there are going to be fatalities.”
The council has underlined the plans are still at an early stage and says it will not comment on every claim made by objectors. The scheme was still at the “outline design and business case stage” with formal consultation not expected to start until next year.
A council spokesman said: “While we fully understand people’s concerns and emotions about the proposed scheme, we cannot avoid the fact that there are some very big issues that we must tackle in the area surrounding the Spa.
“Significant erosion of the toe of the cliff after failure of the sea wall would trigger a large-scale landslide, putting homes and businesses on the Esplanade at risk.
“Although formal planning consultation on the scheme is still some way off, we will be taking all comments on board.”