Protesters are calling on Hampstead’s aspiring MPs to put the protection of Hampstead Heath at the top of their election pledges.

They want Hampstead and Kilburn’s Tory candidate Simon Marcus and his Labour opponent Tulip Siddiq to promise that, if elected, they will lobby for an Act of Parliament to strengthen the existing Hampstead Heath Act of 1871 – designed to preserve the Heath as open space in its natural state.

Around 20 people, armed with slogan banners, gathered at the Mixed Bathing Pond on Saturday as work got under way on the £22million ponds dam project.

They claim that the original Hampstead Heath Act has been violated by Camden Council, who gave the go-ahead to the scheme last month, flying in the face of a 12,465-strong petition against it.

As reported in the Ham & High. in the last few weeks, City of London contractrors began cutting down trees and clearing undergrowth at the model boating pond to prepare for the building of a new spillway.

The work, which will see heavy construction across the Heath and the closure of the swimming ponds, is expected to last until October 2016.

Protesters say the scheme will “vandalise” the Heath and cause “permanent and irreparable damage” to its character.

Leading Saturday’s protest, Sebastian Wocker, who edits the Hampstead Village Voice newsletter, said he was lobbying the candidates “to strengthen the Hampstead Heath Act of 1871, so that this will be the last violation of the Heath”.

He added: “We would like a new, ‘bullet-proof’ Act of Parliament implemented to properly protect the Heath from any more such violations.”

In the meantime, Marc Hutchinson, chairman of The Heath & Hampstead Society, is still waiting for a reply to his letter to environment secretary Elizabeth Truss to intervene by exempting the ponds from laws designed for reservoirs, which means the works would not be necessary.

Officers from the Heath constabulary were at the protest on Saturday to ensure there was no trouble.

A City spokesman said: “The protest lasted for 45 minutes and was peaceful.”