A 450-strong gang of teenagers surrounded the stage door at the Gielgud Theatre, where Radcliffe is starring in play Equus.
Police were called and dispersed some of the fans while Radcliffe was escorted to a car by security guards - but the other two actors remained stranded.
David Pugh, who is producing the play, said the escape bid was not easy for the veteran thespians.
"Richard and Jenny had to get out through the side window by the box office on Rupert Street," he told the Daily Mail. "You try to get Richard Griffiths out of a back window. It was a bit touch and go."
Potter plays odd-minded stable hand Strang in the play, a role in which he appears naked for a sex scene.
Mr Pugh said he was now expecting excited fans of the star but said he hoped they would remain sensible. "It was only after Peter Shaffer approved Daniel's casting that we thought, 'Ooh, what about the Harry Potter fans?'," he recalled. "But touch wood, there has not been a single giggle or whoop or cheer."