Paris was smothered Wednesday by its worst winter pollution in a decade, with commuters enjoying free public transport and half of the cars ordered off the road in an effort to clear the air.

The surge in pollution has been driven by cold weather and near windless conditions that have trapped car exhaust, wood smoke and other pollutants, said the French capital's AirParif air monitoring service.

Although bad by local standards, current levels of fine airborne particles known as PM10 in Paris are around 60 percent of levels in Beijing and a fraction of readings in New Delhi, the world's most polluted capital.

City authorities announced traffic restrictions would be extended to Thursday, with a ban imposed on private cars with registration plates ending in odd numbers from between 5:30 am (0430 GMT) and midnight.

The rules have been in place since Tuesday, alternating between odd and even plates.

Public transport in the city was also free for a second day running to encourage commuters to leave their vehicles at home, while school children are being prevented from exercising outside.

"This is a record period (of pollution) for the last 10 years," Karine Leger of AirParif told AFP by telephone.

For more than a week now, Airparif has published readings of PM10 at more than 80 microgrammes per cubic metre of air particles, triggering the pollution alert.

It recorded the highest level of pollution last Thursday, reporting 146 microgrammes/m3.

Other parts of France were also facing pollution. Officials in the southeast Rhone valley region said they would introduce measures to restrict car use from Friday to tackle the problem in the city of Lyon.

Pollution levels were also high in the Alpine towns of Chambery and Annecy.

- Fines for motorists -

The environmentalist candidate in next year's presidential election, Yannick Jadot, said that politicians needed to target the most polluting vehicles and restrict the use of diesel engines.

"We have politicians who tell us they are looking after our health," Jadot said. "The reality is that when they have to choose between traffic, diesel and our health, unfortunately they don't choose our health."

This is only the fourth time Paris has resorted to traffic restrictions to cope with air pollution. The region's officials took similar measures in 1997, 2014 and 2015.

But a parliamentary report has questioned the efficacy of the restrictions, arguing that they do not target the most polluting vehicles.

Despite the measures on Tuesday, officials reported heavy traffic jams in and around the city in the morning and evening rush hours.

Traffic police were kept busy trying to enforce the anti-pollution measures, fining more than 1,700 motorists for violations.

Paris police chief Michel Cadot warned the traffic restriction might be kept in place for a third day. He called on commuters to limit their car use or organise car sharing to minimise traffic pollution.

Adding to the region's problems, the Paris rail link to its main airport, Charles de Gaulle, has been out of action since Tuesday morning after an accident brought down power lines, forcing more people to take to the road.

State rail company SNCF said Wednesday they hoped to reopen the line by 1500 GMT.

The air particles being measured can cause and exacerbate a range of respiratory and cardiovascular illnesses.