Batman stars Gary Oldman and Katie Holmes came to Mill Hill last Friday night as film crews transformed a medical research centre into the mythical asylum of Arkham in Gotham City.

The stars were on set at the imposing National Institute for Medical Research (NIMR), in The Ridgeway, to film a scene in Batman Begins, which is being directed by Christopher Nolan. Arkham is where Gotham's baddies such as the Joker are locked up.

And an NIMR source said the exposure from the Batman scene could lead to more directors asking to film scenes at the institute.

Massive floodlights loomed over the building, where a mock asylum entrance gate had been built, and black Gotham City police SWAT vans could be seen parked on The Ridgeway. Of the Batmobile, there was no sign.

Amused NIMR staff had been banned from talking about the set but one worker described the scenes which could not be seen from the road.

"It's very exciting it's an asylum. There is lots of rubber stuff and these big drums," she said.

Security was tight and police mounted regular patrols around the perimeter of the site after animal rights protesters threatened to disrupt filming but the solitary protest came from a van driver hooting his horn as he drove by.

Medical experiments on animals are carried out at the NIMR and protesters regularly hold up signs urging motorists to toot their opposition.

Fans Sandy Manuel, 28, of Foscote Road in Hendon, and Jana Manuelpillai, 25, from Wembley, had heard about the filming on the internet.

"I'm just thrilled to be seeing Arkham. It's enough for me you don't have to see Batman himself," said Jana, a fan since the age of 13, proudly pulling out a wallet bearing a silver Batman badge.

"We asked a security guard if filming was taking place and he said no, just as a Gotham City SWAT van drove in behind him. It was classic," said Sandy, who added that he was more of a Superman fan.

It was also a treat for Ian Morris, of nearby St Vincent's Lane, who is a member of the Finchley Cinevideo Society and whose hobby is visiting film sets.

"It's incredible I was just on my way home and I saw all this. It's very serious stuff," he said.

The NIMR is understood to have received a five-figure sum for the use of the site.