Delhi on Sunday introduced a toll for all trucks and commercial vehicles in an attempt to improve air quality in the world's most polluted capital ahead of Diwali celebrations.

Trucks are banned from entering the Indian capital during the day, but every night after 8pm more than 50,000 pour in, according to the Delhi-based Centre for Science and Environment (CSE).

The independent centre says lorries account for nearly a third of the pollution in Delhi, adding to a toxic mix of industrial fumes and dust from construction sites to produce hazardous levels of smog.

Last month, India's Supreme Court approved a four-month trial plan to charge light commercial vehicles an extra 700 rupees ($11) and large trucks 1,300 rupees to enter Delhi.

"The legal principle on which this charge has been levied is the principle of 'polluter pays'," Supreme Court lawyer Harish Salve, whose petition prompted the October 12 order, told AFP.

Successive Delhi governments have faced flak for failing to curb pollution in the Indian capital, whose air quality is worse than even Beijing's.

A World Health Organization study of 1,600 cities released last year showed Delhi had the world's highest annual average concentration of small airborne particles known as PM 2.5.

These fine particles of less than 2.5 micrometres in diameter are linked to higher rates of chronic bronchitis, lung cancer and heart disease as they settle into the lungs and can pass into the bloodstream.

Many trucks transit through Delhi only to avoid paying tolls outside the city, and authorities hope the new toll will encourage drivers to use alternative routes.

- 'Won't solve problems' -

But some have expressed doubts over how effectively it will be enforced, especially after authorities failed to implement a 2001 Supreme Court order banning trucks from passing through the city.

Greenpeace India said the order, which excludes passenger vehicles, oil tankers and trucks carrying foodstuffs, would only divert pollution rather than reduce it, and that drivers would be able to circumvent the toll by using unchecked entries.

"We have to see the effectiveness of it, but this is not going to solve all the problems," campaigner Sunil Dahiya told AFP.

The city is expected to have the world's highest number of premature deaths due to air pollution by 2025 with nearly 32,000 fatalities, according to a study by Germany's Max Planck Institute for Chemistry.

With more than 8.5 million vehicles on Delhi's roads and 1,400 new cars being added every day, city authorities will have their work cut out to reduce pollution.

The city annually experiences a deterioration in air quality when winter sets in as farmers in neighbouring states begin the mass burning of stubble that follows the harvest.

Smoke from open fires lit by the poor to keep warm adds to the problem, while Diwali, the Hindu festival of light, traditionally sees Delhi fill with acrid smoke from firecrackers.

Delhi held its first "car-free day" last week -- although it restricted the initiative to the city centre and chose a public holiday, when traffic would in any case have been much lower than usual.