So-called spycam videos have become increasingly common in the South, where men caught secretly filming women -- in schools and workplaces, toilets and changing rooms -- make headlines on a daily basis.
Distributing pornography is illegal in South Korea, but such videos are widely shared on porn sites and Internet chatrooms or used in adverts for websites promoting prostitution and gambling.
"Those men who film such videos! Those who upload them! Those who watch them! All of them should be punished sternly!" the protestors chanted in unison at the rally held in central Seoul.
Many held banners reading "My life is not your porn" and "We're humans, not a sexual object for your sick fantasy".
Organisers said around 55,000 women took part in the demonstration but police put the estimate at about 20,000.
Most of the protesters were teenagers or those in their 20s -- a demographic seen as the main target of the spycam epidemic.
"I and my friends always look around to see if there are any suspicious holes on a wall or a door whenever we walk into a public toilet stall," a 22-year-old protester who declined to be named told AFP.
"What kind of country has South Korea become? A country where women can't even pee without having to worry about having their butts filmed in secret?"
South Korea takes pride in its tech prowess, from ultra-fast broadband to cutting-edge smartphones. About 95 per cent of its 50 million people possess smartphones -- the highest in the world.
But its technological advances have also given rise to an army of tech-savvy peeping Toms in a male-dominated country with a poor record on women's rights.
The number of spycam crimes surged from about 1,100 in 2010 to more than 6,500 last year, but most offenders were fined or given suspended jail terms described by many women's rights groups as a mere slap on the wrist.
The offenders -- mostly men -- have included college professors, school teachers, doctors, church pastors, public servants, police officers and even a court judge.
Such practices have become so rampant that all manufacturers of smartphones sold in the South are required to ensure the cameras on their devices make a loud shutter sound when taking photos.
But many use special smartphone apps that disable the mandatory sound or use high-tech spy cameras hidden inside eye glasses, lighters, watches, car keys and even neckties.
South Korean President Moon Jae-in said in May that the spycam epidemic had become a "part of daily life" in the country and urged tougher punishments for offenders.
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