NEW DELHI (AFP) - More than a million private cars were banned from New Delhi's roads on Friday (Jan 1), as the authorities began trialling drastic new measures to cut smog in the world's most polluted capital.

From Jan 1, only cars with odd-numbered licence plates will be allowed on the roads on odd-numbered dates and those with even-numbered plates on the other days, except on Sundays when the rule does not apply.

The restrictions will run until Jan 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 United States 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.

On one of the city's main arterial roads, most cars trickling in bore odd-numbered license plates while scores of cyclists wore fluorescent gear, their faces covered.

"Traffic definitely looks thinner today than most days. But I don't know if that's because of the odd-even rule or because people partied too much last night," 58-year-old Mr Mohammad Shahid, a civil defence volunteer, told AFP while standing at one of Delhi's busiest intersections.

Critics have warned the plan could fail in a city where traffic rules are already routinely flouted, with even the federal environment minister calling the move "crazy".

Many believe Delhi residents will deploy the famed Indian skill of "jugaad" - creating a cheap alternative solution - by forging number plates or buying second cars.

But traffic policemen on Friday were happy and surprised to find Delhiites obeying the rules.

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

"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)," he added.

One of the first violators of the new rule at the busy stretch was fined 2,000 rupees (S$42.80) - steep for the average Delhiite - for driving a car with an even-numbered licence plate.

Police asked him to turn around and go back home.

The Delhi government says the car restrictions could be introduced on a more permanent basis if successful, with Chief Minister Arvind Kejriwal promising that he and his ministers would car-pool.

"This plan will only be successful when it becomes a very big movement, when people will want to obey it from their heart," he has said previously, urging Delhiites to cooperate.

"Don't do it because Kejriwal says so, don't do it because the government is forcing you. Do it because you feel it is important for your life, your health."

But some have expressed scepticism about the scheme given the large number of exemptions, which range from motorcycles to women driving alone. Campaigners say motorbikes create up to 31 per cent of total vehicular pollution.

"This odd-even thing isn't going to work," said Mr 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."

Mr Kejriwal's government announced the measures early last December, responding to public pressure to tackle pollution levels more than 10 times the World Health Organization's safe limits.

A 2014 World Health Organisation 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, and another 1,400 are added every day.

The city 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 micrometres 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 US 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.

Delhi's government has hired 3,000 private buses to provide shuttle services into the city from residential areas to cope with the extra ridership during the trial.

Schools have been ordered to remain closed until the trial ends so that their buses can be pressed into action.