One eyewitness who saw the crowds on Bank Holiday Monday, when temperatures in London reached 34.8C, told the BBC there were “massive no swimming signs which were ignored”.
Footage posted on social media also showed a dog in the pond. Dogs are banned from the water through a Hampstead Heath by-law.
The heath is patrolled by Hampstead Heath Constabulary, which enforces by-laws, the City of London Corporation said.
It added that managing the popular open space “is all about finding a balance between the enjoyment of visitors and the conservation of the heath”.