Campaigners fighting to save a historic mansion from demolition have marked a milestone in public activism in presenting a petition with more than 5,000 signatures to the leader of Camden Council.

Cllr Sarah Hayward, council leader, pledged her support to the campaign to stop 19th century Athlone House, on the edge of Hampstead Heath, being torn down after meeting protesters in the town hall foyer on Tuesday night.

Athlone House Limited, which owns the property, wants to replace the Victorian mansion, off Hampstead Lane in Highgate, with an extravagant eight-bedroom home, with a basement swimming pool and ballroom.

The campaign to save it is backed by Monty Python legend Terry Gilliam and more than 5,000 people have joined him in signing a petition calling for Camden Council to refuse the application to demolish the former hospital and RAF intelligence base.

“This is clearly a really important issue in preserving our heritage,” Cllr Hayward said.

“I think we should have more local powers over planning applications and developments. We should be able to preserve this historic building.

“This meeting has demonstrated the strength of feeling around Athlone House and I am grateful to the Highgate Society for organising the petition.”

The 300-page document was handed to Cllr Hayward and cabinet members, councillors Phil Jones and Valerie Leach, by members of the Highgate Society, including chairman Kirsten de Keyser.

“We’ve had many people like English Heritage and Civic Voice saying that this is a milestone in public activism to preserve our heritage,” she said.

As she gave the petition to councillors, Ms de Keyser admitted there are loopholes that can see a scheme approved by a planning inspector after being refused by councils and opposed by residents.

“Why don’t we just say, let’s go out with the old and let’s have something new we can actually work with,” she added.

The protesters are calling on the developers to stick to a 2004 agreement that required the owners to restore the house to its former glory within 42 months of building works on multi-million pound flats in the grounds.

But the council has been unable to enforce the condition because the requirement is frozen while a planning application is being considered.

Cllr Jones said this loophole has allowed for “game playing”.

The petition’s organiser Jack Boswell, 23, of Whitehall Park in Highgate, said: “The [councillors] seemed to be involved and interested in what we had to say.

“It was very satisfying to have a print-out of the petition to give to them,” he added.

An Athlone House spokesman said: “It is unrealistic to expect the new owner to implement the existing planning consent given the very poor condition of the building and the need to provide a very high quality living environment to meet today’s standards.

“The Planning Inspector accepted this reality and concluded that a new house would be acceptable subject to a high standard of design, which we believe [architect] Robert Adam has produced.”