Upskirting is now a criminal offence in England and Wales after a campaign by a woman targeted at a music festival.
Offenders will face up to two years in prison for taking an image or video under somebody's clothing in order to see their genitals or underwear.
Gina Martin, who led the calls for change, said she hoped the change in law would help people "feel comfortable" to report such crimes.
It is already a specific crime in Scotland but not in Northern Ireland.
Speaking on the day the law came into effect, Ms Martin called on people to report upskirting when they see it happen.
She told BBC News: "If a new law's there, great - but if we don't know about it or aren't reporting it, [then] it doesn't do anything.
"We have to build a picture of how much this happens, because it happens a lot."
Prime Minister Theresa May described upskirting as "degrading and humiliating" and said offenders should "feel the full force of the law".
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How the law was changed
Ms Martin, 27, was waiting to watch The Killers perform at the British Summer Time music festival in London's Hyde Park in July 2017 when a man put his phone between her legs and took pictures.
After informing the police, she was shocked to discover upskirting was not a specific offence and the case had been closed.
A few days later she wrote about what had happened on Facebook. Her post went viral with other women sharing similar experiences.
Soon an online petition calling for police to reopen the case had received 50,000 signatures.