The Oscar-nominated actress had just begun her first performance in Thérèse Raquin when the unnamed man allegedly left his seat and walked towards the mezzanine railing to shout down at her on stage.
When Keira didn't break character, he is said to have yelled: "Five seconds is too long to wait for a response."
It's been reported audience members were initially confused as to whether the man was part of the performance. He is then said to have taken his seat while security arrived, allowing Keira and her co-star to continue their performance.
But when security tried to escort him from the theatre, he had one final 'gift' for the star: he threw a bouquet of flowers from the mezzanine to the stage. Audience members have said Keira's co-star kicked the flowers off stage, prompting the audience to applaud. The performance then continued uninterrupted.
A spokesperson for the production has said extra security measures will be taken for forthcoming performances but declined to give details.
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