Seoul: Thousands of South Korean women gathered in Seoul on Saturday to demand stronger government action to fight the spread of intimate photos and footage taken by hidden cameras, which they say has women living in constant anxiety and distress.

Police said about 18,000 took part in the all-women protest, with demonstrators calling for stronger investigations and punishments against male offenders who photograph or film women without their knowledge and post the material online.

Most of the protesters covered their faces with baseball caps, sunglasses and surgical masks as instructed by organisers, who intended to make a statement against an environment in which women constantly worry about tiny cameras hidden in bathrooms or being filmed from under their skirts at subway stations.

However, there was also criticism about the organisers' tight control over the protests and the decision to block the participation of those who weren't "biologically women".

Protesters, many of them wearing red T-shirts with signs that read "Angry women will change the world," roared in support as two women volunteered to have their heads shaved on a stage.

"Let's fight until the last seed of illicit filming dies!" one protester shouted into the microphone while another shaved off her hair. "Here's my message to men: I am not an object of our sexual desires! I am a first-rate citizen, just like you!"

South Korea has struggled over the years to deal with perpetrators who use tiny cameras or smartphones to film under women's clothing to see their genitals or underwear.

The footage is heavily circulated on illicit porn sites, such as Soranet, which had more than a million users before police managed to shut it down in 2016. An unknown number of similar sites are still running.

Since 2004, South Korea has required smartphones to make large shutter sounds when taking pictures and videos to prevent such crimes. However, phone cameras can be silenced through apps and there's also an abundance of miniaturised cameras.