French police have warned Parisians to keep their clothes on or face fines and even imprisonment as residents began flocking to parks and the banks of the River Seine to bask in the summer sun.
In a statement posted on its website, the Paris police prefecture outlined rules for basking in the sun outdoors and at the popular Paris Plages annual event that has sandy beaches and deckchairs laid out on the river bank.
Police said it is not forbidden "when the thermometer rises a few degrees to put on your best bathing suit and find a little corner of grass or the welcoming banks of the Seine to put down your towel."
But it noted that bathing suits should not be worn in official city parks and that dress should be "decent and in accordance with good morals and public order."
Those wearing inappropriate dress could be fined at least 38 euros ($A44) and face fines of up to 3750 euros and two years in jail if suspected of soliciting sex, police said.
And nudity is completely out of the question. "Any outfit that allows for the genital area or breasts to be seen constitutes sexual exhibition and is punishable by a year in prison," police said.