The "quays of the Seine are made for pedestrians", Hidalgo said at the opening of Paris Plages, which will see the road along the banks of the river covered with sand so that tens of thousands of people can sunbathe and play beach games.
Right-wing politicians and some residents have put up a bitter fight against the city council's decision to close the quays permanently to traffic, arguing that it has so far had little effect on pollution levels.
The council argues that the effects will be felt over the longer term.
Hidalgo proudly pointed to a seven-point fall in vehicle traffic in the city since the beginning of the year.
Saturday saw high pollution levels, although not bad enough for an official alert to be sounded.
"It's summer but in Paris we are confronted by unusually long heatwaves," Hidalgo said. "It's the adaptation of the city to climate change that is in the balance here."
She challenged political leaders to have "the courage to take the necessary measures" to fight pollution, a "moral obligation", according to the mayor.
The festival has also spread along the side of the La Villette canal, part of which will be opened to swimmers.