Daniel Trochta, 26, was walking by the Roman Catholic Church of the Assumption of the Virgin Mary in the Czech town of Vranov nad Dyji when he heard terrifying sounds coming from within.
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Unnerved by the screams, Trochta sneaked up to the door and peered through the keyhole - whereupon he suddenly realised he was witnessing a real-life exorcism.
Grabbing his camera phone, he filmed the secret process, capturing people standing close to the curch's altar and the wailing woman.
He then uploaded the footage to YouTube, writing: "I filmed it through a keyhole. I suggest that you use headphones while watching it."
The video, posted on YouTube by Trochta in February, was largely ignored until Catholic Church priest Marek Dunda confirmed it was genuine last week.
He said: "If something is filmed through a keyhole, there is nothing more to say about it.
"We were asking God to protect and to liberate the person. We used a Latin prayer asking for a help."
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