Paris on Monday resorted to drastic measures to curb sky-high pollution by banning all cars with even number plates for the first time in nearly two decades.

In a move that infuriated motorist organisations, around 700 police officers were deployed to man 60 checkpoints around the French capital to ensure only cars with plates where numbers end with an odd digit were out on the streets.

Public transport has been free since Friday to persuade Parisians to leave their cars at home, and at rush hour on Monday morning, authorities noted there were half the usual number of traffic jams as drivers grudgingly conformed to the ruling.

Some, though, appeared unaware of the restrictions that came into force across Paris and 22 surrounding areas from 5:30 am (0430 GMT) -- or chose to ignore them.

"You don't have the right to drive with your number plate," a man on a scooter remarked to another while stopped at a red light.

"Oh really? I didn't know," the second driver replied before speeding off.

The restrictions will be reviewed on a daily basis, with odd numbers potentially banned on Tuesday if deemed necessary -- a decision due to be made later in the day.

The government decided to implement the ban on Saturday after pollution particulates in the air exceeded safe levels for five straight days in Paris and neighbouring areas, enveloping the Eiffel Tower in a murky haze.

On Monday, Airparif, an official monitor for air quality in Paris and neighbouring areas, said pollution levels had since fallen.

"There are encouraging signs that it is decreasing," said Anne Kauffman, Airparif's number two.

Those who chose to defy the ban risked a fine of 22 euros ($30) if paid immediately, or 35 euros if paid within three days.

By midday Monday, Paris police said they had doled out nearly 4,000 fines to drivers not respecting the restriction.

Electric and hybrid cars, as well as any vehicle carrying three people or more, are exempted from the ban -- the first since 1997.

- Ban is 'hasty, ineffective' -

The issue has become something of a political football, with less than a week to go before key municipal elections.

The opposition UMP candidate for Paris mayor, Nathalie Kosciusko-Morizet, called the measure a "fig leaf".

Ecology Minister Philippe Martin said he understood the "difficulties, the irritation and even anger" over the move, adding: "But we just had to take this decision."

Martin said similar measures in 1997 "had yielded results."

But others complained that the free public transport came at a cost.

Socialist party member Jean-Paul Huchon, who is also head of the STIF organisation that oversees transport in Paris and neighbouring areas said STIF stood to lose 4 million euros a day.

France's Automobile Club Association (ACA), which counts some 760,000 members, denounced the move as "hasty, ineffective."

"This measure had no effect in any country where it was introduced," said ACA head Didier Bollecker.

"Drivers are being targeted even though heating is more polluting, but no one is asking for heating to be used on alternate days."

- Global idea -

Similar measures were used in Beijing during the 2008 Olympics, and the result was so successful that the city continues to apply them once a week.

In Paris, authorities measure the concentration of particulates with a diameter of less than 10 microns -- so-called PM10 -- in the air to determine pollution levels.

PM10 are created by vehicles, heating and heavy industry, and include the most dangerous particles that measure less than 2.5 microns in diameter, which can penetrate deep into the lungs and the blood system and are cancer-causing.

The safe limit for PM10 is set at 80 microgrammes per cubic metre (mcg/m3). At its peak last week, Paris hit a high of 180 mcg/m3 but this had fallen to 75 mcg/m3 by Monday.

According to a 2011 World Health Organisation report, the planet's most polluted city was Ahvaz in Iran with an average of 372 microgrammes per cubic metre.

Beijing had an average of 121 mcg/m3, while Paris was measured at 38 mcg/m3.