Since dawn many of the cities' streets have been closed to private cars that run on petrol or diesel, encouraging people to use public transport, bicycles and even rollerblades.

The restrictions will be lifted at dusk.

The BBC correspondent in Paris says the scheme is having a mixed success there: some central areas are closed, but most of the city is working normally.

Our correspondent says that many drivers have failed to heed the call to leave their cars at home and have caused long traffic jams as they take detours to avoid the restricted areas.