HELEN Mirren broke character from her regal role when she yelled at a group of drummers to be quiet while still dressed in her stage costume as Queen Elizabeth II.
The 67-year-old, whose performance in The Audience won her an Olivier Award last week, left London's Gielgud theatre to confront the drum troupe.
As One in the Park, who were promoting the festival for gay and transgender people due to take place in London this month, said 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 for a drum crescendo, was quiet after the telling off.
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."
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, it was reported.