Dame Helen royally routs drummers
The 67-year-old, whose performance in The Audience won her an Olivier Award last week, left London's Gielgud theatre to confront the troupe of performers.
As One in the Park, who were promoting their festival for gay and transgender people due to take place in east London this month, said Dame Mirren was dressed in pearls and a tiara when she demanded the noise stop.
A statement from the festival read: "Clearly angered she shrieked 'Quiet! I'm trying to do a play in here! People have paid a lot of money for tickets'."
Rufus Wright, who plays Prime Minister David Cameron in the production, also told the musicians to be quiet. On Twitter he wrote: "Just fulfilled a lifelong ambition by bellowing at 25 drummers to shut the f*** up. West End theatres got thin walls."
He added: "You should have seen Helen. She came out in full Queen costume and shouted at the drummers too. Honestly. It was breathtaking".
The parade, which had paused outside a bar on Rupert Street for a drum crescendo, were quiet after their telling off from Dame Helen.
Mark Williams from festival organisers Orange Nation said: "Obviously we are terribly upset if we caused her any distress. If she'd like to let her hair down and attend the festival she'd be more than welcome."
According to The Telegraph, the noise from the parade became increasingly difficult to ignore towards the end of the first act and during the interval, audience members discovered a band of drummers outside the theatre.
Ben Scotchbrook, a television news presenter who was watching the play, told The Telegraph he could not find a policeman to intervene and was unable to move on the group himself. But after Mr Scotchbrook returned inside, Dame Helen appeared from the stage door and was more successful in quieting the incessant noise.