The move by authorities in the French capital seeks to cut health-endangering pollution days before town hall elections.
Amid concerns of worsening air quality after a week when unseasonally balmy weather boosted pollution, public transport was free of charge, and drivers with even-numbered licence plates were told to leave their cars at home of face fines.
Paris is more prone to smog than other European capitals because of France's diesel subsidies and its high number of private car drivers.
Some 700 police manned key entry points to the city from before dawn to apply the scheme, in which drivers may only use their cars on alternate days, depending on whether their licence plates finished with an odd or even number.
Transport Minister Frederic Cuvillier said early results showed traffic tailbacks had been shrunk by 60% during yesterday's morning rush hour and the ban may be repeated.