Residents are complaining of headaches, coughs and smarting eyes. All 6,000 schools in Delhi are closed till the end of the week. A Delhi government advisory has urged anyone with breathing difficulties to remain indoors and said everyone should avoid strenuous activity.

The Delhi government has decided to reintroduce the "odd-even" scheme for Monday-Friday under which cars with licence plates ending in an odd number are allowed one day and even-numbered cars the next. Women drivers and two-wheelers are exempt; violators will be fined Rs 2,000. Thousands of traffic policemen and volunteers will enforce the restrictions.

PM 2.5 is particulate matter about 30 times finer than a human hair. The particles can be inhaled deep into the lungs, causing heart attacks, strokes, lung cancer and respiratory diseases. Delhi registered levels of over 600 today - 12 times over the safe limit.

Illegal crop burning in neighbouring Punjab and Haryana, vehicle exhaust emissions in a city with limited public transport, and swirling construction dust have caused the crisis, which arises every year.

The Delhi High Court has ordered an emergency meeting within three days between the top bureaucrats of the central government and those of Delhi, Haryana and Punjab. Representatives of agencies that handle pollution are also to attend.

"I'd like to assure people that the central government shall do everything possible to bring about improvement in air quality in Delhi and the Nation Capital Region," said Environment Minister Harsh Vardhan as authorities faced criticism for failing to take steps to fight a problem that erupts every year.

Late yesterday, the Delhi government announced a set of measures to try to clean up the air. Commercial trucks have been banned from the city unless they are transporting essential commodities, all construction has been stopped and car parking charges raised four times to force residents to use public transport.

"The situation as it exists today is the worst that I have seen in my 35 years staying in the city of Delhi," said Arvind Kumar, a lung surgeon at Sir Ganga Ram Hospital. "As a doctor, I have no problem saying that the situation today is a public health emergency. If you want to protect people, we should be ordering the evacuation of Delhi. Closing down all schools. Closing down all offices."

Farmers in Haryana and Punjab, the two big agrarian states surrounding Delhi, regularly defy a ban to burn millions of tonnes of crop waste around October every year before sowing the winter crop of wheat. State authorities say it is hard to enforce the ban unless farmers, a powerful political constituency, are given funds to buy machinery to clear their land.