The court has been spearheading a drive to end choking pollution in New Delhi, rated one of the world's dirtiest cities where doctors report a mounting toll of people with breathing ailments.

In 1998 it ordered that all buses switch to CNG and set a three-year deadline.

More than 6,000 buses, half the number that normally ply the streets, were taken off the road. Those still running were stuck in traffic jams.

"The bus came after an hour and then there was no place so we had to walk for an hour," said one exasperated commuter.

Police were braced for violence following threats by some private operators to disrupt partial services. "Various reactions can be expected ranging from road hold-ups, mob violence and rioting," deputy police commissioner S.B.S. Tyagi said.

Last year, enraged commuters torched buses to protest against lack of public transport following a similar court order.

The court had extended at least twice the deadline for phasing out diesel buses and said enough time was given to bus operators to stop using diesel, which it said was a dirtier fuel.

The court backed up its ruling on Friday by saying transit operators would have to cough up 500 rupees ($10.26) a day for each diesel bus on the road. The fine would be doubled to 1,000 rupees a day after a month.

Bus operators say they cannot afford the fine.

In a toughly worded judgement, the court said "the priority must be public health as opposed to the balance sheet of a private company", and quoted estimates that the health cost of air pollution in New Delhi was 10 billion rupees.

But operators said the city administration had not provided enough CNG outlets to allow them to switch fuels.

Local media reports said public transport operators would ask the government to bring in a decree allowing use of low-sulphur diesel as another option to CNG.