The burkini ban imposed in parts of France has been condemned by London Mayor Sadiq Khan during a visit to Paris.
Mr Khan said women should not be told what to wear as protesters campaigning against the ban staged a beach party outside the French Embassy in London.
The protesters, some carrying banners warning against Islamophobia, put up windbreaks and built sandcastles outside the embassy while campaigning under the slogan "wear what you want".
Mr Khan, who is a Muslim, waded into the row as he travelled to the French capital on his first official overseas visit.
"I'm quite firm on this. I don't think anyone should tell women what they can and can't wear. Full stop. It's as simple as that," he told the Evening Standard.
"I don't think it's right. I'm not saying we're perfect yet, but one of the joys of London is that we don't simply tolerate difference, we respect it, we embrace it, and we celebrate it."
In London, women gathered at a makeshift beach to protest against the ban by some French authorities on burkinis, a type of tunic with a headdress worn by some Muslim women to keep their bodies covered on the beach.
In France there are strict rules over religious dress.
Images of uniformed police appearing to require a woman to take off her tunic on a beach in Nice led to online anger about the ban, triggering the protest.
Critics who described the ban and the image of the police officer as "humiliating", gathered at the embassy, where they built a makeshift beach and wore different types of swimwear.
Organiser India Thorogood, 25, who planned the event less than 24 hours in advance via a Facebook group, said it is "unacceptable" that men should tell women what to wear, and that French authorities are specifically "targeting" Muslim women.
But the support was not unanimous, as the display was interrupted by a woman who stormed the "beach" and complained that campaigners for women's rights ought to focus more on education.
Nice was the scene of a bloody terrorist attack in July and the city's deputy mayor Rudy Salles defended the ban.
He said the burkini was "a provocation", telling BBC Radio 4's World at One: "When you see a burkini on the beach, people feel unsafe."
Mr Salles added that Catholic nuns would not be allowed to wear their habits on the beach in the resort.