The US Embassy air quality index, which measures the concentration of tiny PM 2.5 particles, exceeded 500, indicating serious aggravation of heart and lung disease, and premature mortality in people with existing diseases and the elderly.

Pollution at this level also means serious risk of effects on the respiratory systems of the general population.

The city government has declared a public health emergency, and imposed an "odd-even" system on private vehicles, at least until November 15.

On Monday, drivers with even-numbered licence plates were the lucky ones. Morning traffic was thin and drivers appeared to be obeying the rule - a Reuters reporter saw no vehicles with odd-numbered licence plates on the streets.

"It' a huge inconvenience because I'm not going to make it on time for my meetings," said Sagar Bajaj, 29, struggling to find a taxi in central Delhi's busy Connaught Place.

Bajaj said he normally drives to work but his car's licence plate ends in and odd number.

Ride-hailing services were exempt from the rule and both Uber and Ola had announced they would not impose surge pricing for the duration of the odd-even scheme.

Vehicular exhaust along with emissions from industry contribute more than 50 per cent of Delhi's air pollution on most days through the year, according to official estimates.

The city also ordered schools shut on Monday.

Authorities have also ordered all construction work to stop.

A government monitor on Sunday showed air quality had hit the worst level for the year, at 494 on a scale of 500. The level was well above 400 early on Monday.

According to independent online air quality index monitor AirVisual, New Delhi was the most polluted major city in the world on Monday, at twice the level of Lahore in Pakistan, which was a distant second.

Australian Associated Press