With the city effectively shut down until Saturday, police were out in force to prevent motorists from entering a large part of the city without a permit. Officials hope that will help clear the suffocating blanket of brown-yellow haze.

"The citizens who violate today's expanded ban on entering restricted traffic areas will be fined," Tehran's traffic police chief, General Hossein Sajedinia, told state television.

He said restrictions will continue as long as the pollution alert is on.

Health alerts are becoming increasingly common in the city of around 10 million people, where the air is deemed unhealthy for at least 100 days of the year. Complaints of asthma, allergies and respiratory ailments are also on the rise.

This week the smog is denser than usual due to a total lack of wind, and schools have already been closed and the sick and elderly told to stay indoors.

Many of the two million plus vehicles in the city are more than 20 years old and consume cheap subsidised petrol at an alarming rate. Private car ownership has also exploded, with the public transport system failing to provide adequate coverage.

The government has proposed various steps to resolve the problem, such as phasing out the old cars, mandatory emissions checks and restricting vehicle use on certain days of the week. So far, none has been effectively applied.

"If the pollution continues at this level, the rule of driving on certain days of the week based on even and odd plate numbers of cars will put in force," Sajedinia said.

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