The anonymous letter, shown in a report by Tele Zürich, said: “We know what you've done, who with, how long for and for what price. But you know that too! We hope that you haven't forgotten; if so we can refresh your memory with some photos.”
The author of the letter gives his target five days to pay 1,999 francs in Bitcoin.
Among those affected are around 50 customers of Globe, a sex club in Schwerzenbach, in the canton of Zurich.
Speaking to Tele Zürich Fritz Müller, the manager of the club, said the letter is an empty threat since no such photos could exist.
Photography is forbidden in the club, as are mobile phones, he said.
A police investigation is ongoing.