PARIS (AFP) - Paris on Monday resorted to drastic measures to curb soaring pollution levels by forcing all cars with number plates ending in even numbers off the road for the first time in two decades.

Around 700 police officers were deployed to man 60 checkpoints around the French capital to ensure that only cars with number plates ending in odd numbers were out on the streets.

Public transport has been free since the weekend to persuade Parisians to leave their cars at home, and the state railway company SNCF warned on its website of packed suburban trains at peak hours due to the extreme measure.

"It is sure we will have more clients today," a delighted taxi driver told AFP.

"There are people who take their car because they don't want to be pressed up against others in the metro. Today they will take a taxi."

The restrictions came into force across Paris and 22 surrounding areas from 5.30am (12.30pm Singapore time).

They will be reviewed on a daily basis, with odd numbers potentially banned on Tuesday if an extension is deemed necessary.

Parking will be free for vehicles with even number plates, the Paris city hall said, calling on residents to consult carpooling or car-sharing sites to work out their travel plans.

However not everyone seemed to be aware of the ban, or chose to ignore it.

"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.

Those who choose to brave the ban risk a fine of 22 euros (S$38.70) if paid immediately, or 35 euros if paid within three days.

Electric and hybrid cars will be exempted from the ban as well as any vehicle carrying three people or more.

It is the first time since 1997 that the French authorities have resorted to such a drastic measure.

The government made the announcement on Saturday after pollution particulates in the air exceeded safe levels for five straight days in Paris and its environs.

One enterprising website did not waste any time cashing in on the restrictions. Lending site e-loue.com encouraged Parisians with odd-number plates to rent them to neighbours who could not drive on Monday.

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," adding that he hoped that the number of vehicles on the roads would be "significantly lower" on Monday, without giving a figure.

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

"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." By Saturday the number of pollution particulates in the air had fallen slightly after hitting a high of 180 micrograms per cubic metre - well over double the safe limit - on Friday.

So-called PM10 particulates are created by vehicles, heating and heavy industry, with the safe limit set at 80 per cubic metre.

The smoggy conditions have been caused by a combination of cold nights and warm days, which have prevented pollution from dispersing.

The pollution particulates in the air can cause asthma attacks as well as respiratory and heart problems.

The World Health Organization has said finer particulates - known as PM2.5 - are cancer-causing.