Paris has cut the speed limit on its roads to only 30kph (19mph) to help protect the climate and create safer streets for pedestrians.
The French capital, noted for its traffic congestion and car-clogged streets, is often blighted by severe pollution in summer.
Officials hope the move will make the city more pedestrian-friendly and encourage more people to ditch their cars, thereby reducing emissions.
However, some motorists, including delivery and taxi drivers, say the plan will lead to more congestion and affect their businesses.
Pierre Chasseray, a campaigner with the 40 Million Motorists group, challenged the idea that noise and pollution would be reduced.
"There is no reduction in sound, there is no reduction in pollution and there is no reduction in accidents, except for a reduction in accidents which is the same as in all the other communes," he told AFP news agency.
But polls suggest that residents broadly welcome the idea in the hope it can reduce pollution and noise.
Under Socialist Mayor Anne Hidalgo, the Paris city government has already restricted or banned vehicles on several streets and increased the number of bike lanes.
Already, cyclists often move faster than cars in the densely populated inner city.
The new limit includes exceptions for a handful of wide avenues including the Champs-Elysees and the bypass circling the French capital.
Other French cities and towns already have a 30kph speed limit, such as Montpellier and in much of Grenoble, Lille and Nantes.