Paris (AFP) - French authorities forced half the cars off the roads of Paris on Monday in a bid to reduce spiking smog levels over the past week.

All cars with licence plates ending in even numbers were taken off the roads of the capital and its suburbs, but plans to extend the measure to Tuesday for cars with odd numbers were cancelled after forecasts showed pollution levels falling.

Public transport was free of charge to take the sting out of the restrictions, but ferociously independent drivers in the capital have still railed against the measure, despite it having been used only twice since 1997.

Similar emergency restrictions were last implemented almost exactly a year ago -- on March 17 -- during a particularly bad spike in the pollution levels.

The Airparif monitoring agency said smog density was still near alert levels Monday, but rain forecast for Tuesday was expected to clear the air -- allowing government officials to halt the restriction after a single day.

"Due to the improving situation today and tomorrow, the alternative traffic (plan) will not be renewed Tuesday," announced Ecology Minister Segolene Royal, who saluted "the good citizenship of motorists who understood the necessity of this measure."

Not all motorists with even-numbered plates were as cooperative as Royal suggested, which explained the presence of 750 police officers mobilised in and around the capital to keep watch for defiant drivers.

"I work -- I don't have time to lose with this," said a man stopped at a red light with even-numbered plates, when asked why he'd ignored the restriction and risked a 22 euro ($24.5) fine.

The measure was also criticised by environmentalists for coming too late following a full week of high pollution levels.

"You must not confuse speediness with undue haste," Royal responded.

The Paris area is confronting an increasingly troubling air pollution problem, caused by vehicles, heating and heavy industry, which generate tiny floating particles that can penetrate deep into the lungs and the blood system and can cause cancer.

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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.

The safe limit for PM10 is set at 80 microgrammes per cubic metre (mcg/m3), with levels in Paris Monday expected to be somewhere between 40 and 55 mcg/m3. The city's alert level is fixed at 50 mcg/m3.

According to a 2011 World Health Organisation report, the planet's most polluted city was Ahvaz in Iran with an average of 372 mcg/m3.

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