Police said more than 4,000 people were hit with fines in Frances' capital by midday on Monday, just hours into the drastic government measure.

From 5:30 am local time (0430 UTC) police were told to pull over all vehicles whose number plates ended on an even number.

All electric and hybrid vehicles, taxis, ambulances, driving school cars, refrigerated trucks and vehicles were exempt.

Motorists, who either forgot the ban or were in flagrant violation of it, were ordered to pay an immediate 22-euro ($30) fine.

Around 700 police cars at 60 checkpoints around the city and in 22 surrounding areas to enforce the day-long ban.

There was the question of continuing the restriction into Tuesday, alternating the vehicles affected.

But France's Ecology Minister Philippe Martin said Monday's ban had already "led to results" showing "a clear trend of improvement," making a second day of restrictions unnecessary.

He thanked the motorists for their cooperation, saying "90 percent of the cars on the city's roads today had odd number plates."

Electoral sore point

The ban, the first of its kind to be introduced since 1997, was ordered by the French government over the weekend in an effort to reduce soaring smog levels.

Pollution particles in the air had exceeded safe levels for five straight days, cloaking famed monuments like the Eiffel Tower in a murky haze.

The European Environment Agency released figures on Thursday showing 147 micrograms of particulate matter per cubic meter of air in Paris - compared with 114 in Brussels, 104 in Amsterdam, 81 in Berlin and 79.7 in London.

The cities of Lyon, Caen, Rouen, Grenoble were among others to have been affected.

In an effort to get motorists behind the measure, public transport was free in Paris from Friday until Monday evening.

Nevertheless, the highly controversial measure, which came six days before the start of municipal elections, faced significant criticism.

The opposition UMP candidate for Paris mayor, Nathalie Kosciusko-Morizet, described the measure as a "fig leaf," while others complained free transport came at a cost.

According to Socialist party member Jean-Paul Huchon, who heads the STIF organization that oversees transport in Paris, STIF was facing losses of 4 million euros a day during the four-day free transport period.

ccp/hc (AFP, Reuters)