Undaunted, they have called on Facebook for a third "operation burkini" this Sunday, claiming they are the "Muslim Rosa Parks" of France, a nod to the US civil rights icon.
In response to the high-profile action, Grenoble residents have launched a rival "citizens' secular, ecologist" Facebook group called "Everyone Naked", calling on all members to turn up to the pool "baring all" to take on the "burkini brigade".
They accuse the Green mayor, Eric Piolle, of "worrying inaction".
Some 231 people say they will come naked and a further 1,300 have expressed an interest. Among those is a Socialist regional councillor, Stéphane Gemmani, who confessed he was still in two minds whether to show up "nude or not".
Caught in the crossfire, lifeguards downed tools this week saying they weren't able to focus on their job and the two pools were shut despite the heatwave.
"We are working towards a positive solution" to the problem, said the town hall, which has made it clear it has no intention of lifting the burkini ban despite the heatwave.
The pro-burkini group said it had decided to act after 630 people signed a petition asking for the rules to be changed so Muslim women could bathe in public baths.
Adrien Roux, national head of the Citizen's Alliance, said: "(The mayor) hasn't uttered a word on the fact that due to their religious beliefs, hundreds of Muslim women can't go swimming. Republican principles should protect their freedom of conscience: a public service must be open to all," he told Le Figaro.
"Some think these are Islamists who want to wreak havoc in France. They are just asking for the rules to take into account diversity" as is the case in the town of Rennes, northern France, or Germany and the Netherlands, he said.
But Amine El-Khatmi, president of secularist group Republican Spring, retorted: "These are not poor Muslim mums wilting in the heat but political militants at work."
"Next time, it will be: 'We want to book the whole pool because we can't bathe with others'," she warned.
Matthieu Chamussy of the opposition Right-wing Republicans party, said: "Political Islam is moving forward step by step and the cause of women receding."
Marlène Schiappa, the gender equality minister, was more nuanced.
She said she was against "community segregation" by a "tiny minority" and that the message behind was to "create a new norm: cover yourself". But she added that the debate should not lead to discrimination and that "women, whatever their religion or way of life, should be able to access municipal baths".
The mayor said the row had gone over his head and called for the state to intervene to "remove all ambiguity on the status of body-covering swimming costumes".
France - the country with Europe's largest Muslim population - was the first European country to ban the full veil in public spaces in 2011. However, it is up to municipalities to fix the rules on the burkini.
The garment was at the heart of a standoff in several French seaside towns three years ago. Some towns banned the garment, claiming it was a security threat, only to have the bans later overturned by a court.
