Teenage film star Daniel Radcliffe threw off Harry Potter's invisibility cloak today to appear naked on a London stage in a revival of the psychological thriller Equus.

Radcliffe, who found fame as the young wizard in the Potter films, plays a troubled stable boy who ritualistically blinds horses in a production that has caused a flurry of excitement among his hordes of teenage fans.

He won a standing ovation on the opening night at London's Gielgud Theatre and fellow actors who came to watch were full of praise for his courage in taking on such a gruelling role.

"I am so impressed with his choice of play and the career Daniel Radcliffe is having," Hollywood star Christian Slater said.

You could hear a pin drop in the auditorium when 17-year-old Radcliffe performed a much-talked about naked scene with actress Joanna Christie, who plays his girlfriend.

"It is an incredible part for a young actor. He is taking a big risk," British actor Richard E. Grant said.

Not since Nicole Kidman bared all in The Blue Room in 1998 and was hailed as "pure theatrical Viagra" has so much hype greeted stage nudity in London with publicity photos of Radcliffe helping to ensure ￡2 million pounds in advance sales.

Pottermania struck at the stage door with fans of the actor clamouring for autographs at previews of Peter Shaffer's play, first produced in 1973 to much critical acclaim.

Radcliffe, seeking to make the difficult transition from child star to adult actor, had no compunction about stripping off for the role.

"Equus is an iconic play. The nude scene is part of it. I can't do it with my pants on. That would be rubbish," he has said of his part as troubled teen Alan Strang.

But Radcliffe, who has had voice coaching and worked out in the gym in preparation, at least has one familiar face on stage with him in Equus.

The role of the psychiatrist who treats the tortured boy is performed by Richard Griffiths, the portly British actor who plays Potter's vile Uncle Vernon in the Potter films.

But all the hype has irritated some critics.

"With Radcliffe posing for peek-a-boo pictures that stop just short of his pubic hair, the press and publicity machine seem to be engaged in some kind of mad titillatory game," said The Guardian's critic Michael Billington.

Alistair Smith, news editor of The Stage weekly newspaper, said all the publicity and advance sales "have made the production all but critic-proof".