YOBS hurled lightbulbs towards the stage at York Theatre Royal, but could not stop the show going on.
Police had to be called to the theatre after a group of youths struck during the opening night of the English Touring Theatre's production of Mother Courage And Her Children.
Vicky Biles, Theatre Royal general manager, said: "Some people got into the gallery level of the theatre in the second interval and objects were dropped down. Members of staff were alerted and they called the police, who are now investigating.
"It's a very sorry state of affairs when something like this happens. I have no idea why anyone would do it.
"Thankfully it wasn't worse, but nevertheless we do take security issues like this very seriously."
The incident happened at about 9.30pm on Tuesday, two hours into the production, when what were believed to have been lightbulbs were thrown at the lowered security curtain. The youths ran off, but the show went on.
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