More than 1,500 hotel guests were secretly filmed with footage of them live-streamed to the internet, according to police in South Korea.
The illicit activity was discovered in in 30 separate hotels across 10 South Korean cities.
Cameras were hidden in digital TV boxes, wall sockets and hairdryer holders, and the footage streamed to a site with more than 4,000 members.
Four men have been arrested.
The footage was sent to a private site, where members paid a monthly fee to access the videos, it added.
It is not the first time that spy cameras have been uncovered in South Korea.
In 2017, more than 6,400 cases of illegal filming were reported to police.
Last year, tens of thousands of women took to the streets in cities to protest against the practice, under the slogan "My Life is Not Your Porn".
The spy camera problem has led police in Seoul to launch a special squad of female inspectors who regularly visit the city's public toilets looking for spy cameras.