Authorities banned traffic on Sunday from more than 100 towns and cities across Italy, including Rome and Milan, in an effort to cut growing pollution levels, city officials said.
Partial blocks on cars and lorries have already been imposed in recent days in many places after health departments warned that prolonged calm weather had allowed smog to build.
With no improvement in sight, mayors decided they had to force car-mad Italians off the roads with a one-day ban.
"We had no alternative," newspapers quoted Rome mayor Walter Veltroni as saying.
"We now have new (pollution) limits imposed on us by the regions who are following European norms and the weather conditions are not helping to disperse the smog."
All non-essential traffic was banned from the inner-Rome area from 10.00 am to 5.00 pm (0900-1600 GMT).
However, Environment Minister Altero Matteoli said the one-day ban would have little impact and said an alternative solution was to encourage people to scrap their old cars and scooters and buy more environmentally-friendly vehicles.
"Forcing people to walk once in a while is of little use," he said in an interview with Il Messaggero newspaper.