Tens of thousands of South Korean women on Saturday protested against secretly-filmed spycam pornography as anger over the issue swells, prompting national soul-searching.

More than 40,000 women took to the streets of Seoul yesterday, as part of a monthly demonstration started in May.

The movement has shattered records to become the biggest-ever women's protest in South Korea where the global #MeToo movement has unleashed an unprecedented wave of female-led activism.

The target of their fury is so-called "molka" or spycam videos which involve men secretly filming women in schools, offices, trains, toilets and changing rooms. This crime is so prevalent it makes headlines in South Korean media on a daily basis.

"Entering a public bathroom is such an unnerving experience these days," protester Claire Lee said, adding that she always looked around the walls to see if there were any "suspicious holes".