More than a million private cars were banned from New Delhi's roads on Friday, as authorities began trialing drastic new measures to cut smog in the world's most polluted capital.

From January 1 only cars with odd-numbered license plates will be allowed on the roads on odd-numbered dates and those with even-numbered plates on the other days to try to reduce pollutant levels which regularly hit 10 times the World Health Organization safe limits.

The restrictions will run until January 15 on a trial basis as part of a wider drive aimed at reducing pollution levels that also includes shutting some coal-fired power plants and vacuuming roads to reduce dust.

As the restrictions came into force on Friday morning, pollutant levels hit a "hazardous" 429 on the U.S. embassy's air quality index, meaning everyone is at risk of respiratory problems and children and older people should stay indoors.

Hundreds of traffic police and volunteers took to the streets to enforce the scheme, including dozens of children wearing smog masks and holding banners urging drivers to comply.

Most drivers appeared to be adhering to the rule with Delhi's usually clogged roads flowing relatively freely.

"Delhi has done it!" tweeted the city's Chief Minister Arvind Kejriwal, who said he was carpooling with colleagues to get to work.

"Reports so far v encouraging," he added.

One early violator caught driving a car with an even-numbered license plate on a busy stretch of road was fined 2,000 rupees ($30) and ordered to turn around and return home.

"I would have expected to catch at least dozens in the first half an hour, but surprisingly most people are obeying," said Ankit Kumar, a traffic policeman.

"At least over here which is usually a pretty hectic intersection. This is a good sign. But let’s see what happens on Monday (when more commuters hit the roads)."

Schools have been ordered to remain closed for the 15-day period so that their buses can be used to ferry commuters to work.

The Delhi government says the measures could be introduced on a more permanent basis if successful, although some believe city residents could try to get around the restrictions by forging number plates or buying second cars.

Critics say the restrictions do not go far enough, with motorcyclists and women driving alone exempt. Campaigners say motorbikes create up to 31 percent of pollution from vehicles.

"This odd-even thing isn't going to work," said Kirti Lal, who commutes by bus.

"Just wait for Monday, people are going to be back to their old habits. Delhiites are too used to their cars," he told AFP.

A 2014 WHO survey of more than 1,600 cities ranked Delhi as the most polluted, partly because of the 8.5 million vehicles on its roads.

Just under three million of these are private cars or vans, but car sales are soaring as incomes rise, with 1,400 extra vehicles pouring onto the city's already crowded roads every day.

More than 23 percent of the cars on Delhi roads run on diesel, which is cheaper than cleaner petrol, according to the Delhi-based Center for Science and Environment think-tank.

India's capital has been shrouded in a toxic blanket of smog in recent weeks as winter sets in and cooler temperatures trap pollutants in the atmosphere, pushing harmful PM 2.5 levels sky-high.

These fine particles less than 2.5 micrometers in diameter are linked to higher rates of chronic bronchitis, lung cancer and heart disease after settling into lungs and passing into the bloodstream.

On Friday the U.S. embassy figures put PM 2.5 levels at 264 in Delhi, well above the WHO safe limit of 60.

In a measure of mounting concern, India's Supreme Court has recently ordered a temporary ban on large new diesel cars in Delhi and doubled a tax on diesel trucks.