SHRIEEEEK! At a chi-chi French Chateau mansion in Beverly Hills, filming on a commercial for KATY PERRY’s new oo-la-la French fragrance suddenly came to a screeching halt when the elegantly-gowned superstar burst from the master suite, screaming: “A man in the mirror!...Staring at me!...A GHOST!!”

As producers rushed to help panicked Katy, security guards raced to the second floor of the swanky mansion – once owned by an eccentric Saudi Arabian who threw lavish parties there until he died and his heirs put it on the market for a staggering 68 million buck$$.

Said My SpyWitness: “Shivering and terrified, Katy told producers that while she was alone in the master suite, she turned to the mirror – and froze when she saw a man sporting a mustache staring back at her, his face contorted in agony.(OMG! RUSSELL BRAND??!)

"Katy raced from the room and down the enormous staircase, screaming at the top of her lungs! Everyone kept trying to calm her down, until security came downstairs and told producers they’d found nothing!”

Everyone kept suggesting that what she’d seen was probably just a reflection – but the more they tried to brush it off, the more upset Katy became.

Said the source: “She told them she would NOT return to that room – and wanted to get out of the house immediately!” Finally, the director took Katy aside, soothed her by saying they’d cover up every mirror and keep a security guard by her side every minute – so the shaken star bravely went back to work! (Merde!)