NEW DELHI — Air quality in India's capital slid deep into the 'very poor' category on Sunday as Diwali celebrations got under way. Fireworks joined crop-burning smoke and vehicle exhaust over a city of more than 30 million people.

The Central Pollution Control Board's daily bulletin placed Delhi's 24-hour average Air Quality Index above 300 by late afternoon, before the main evening festivities began. Monitoring stations in Anand Vihar, Jahangirpuri and Wazirpur recorded some of the worst readings. Concentrations of fine PM2.5 particles there ran at many times the World Health Organization's recommended limit.

Readings rose sharply after dark as fireworks went off across residential neighbourhoods, despite long-standing restrictions on their sale and use in the capital. Calm winds and falling night-time temperatures trapped smoke close to the ground.

The India Meteorological Department had forecast that these conditions would slow the dispersal of pollutants into Monday. Satellite data from the Indian Agricultural Research Institute also continued to show farm fires in Punjab and Haryana, where stubble burning adds smoke to the region's air each autumn.

The Commission for Air Quality Management, which oversees pollution controls across the National Capital Region, said it was monitoring conditions closely. It could invoke tighter measures under its Graded Response Action Plan if levels entered the 'severe' band. Such measures could include curbs on construction and on some diesel vehicles.

Doctors at the All India Institute of Medical Sciences urged children, older people and those with respiratory conditions to stay indoors and limit exertion.

The Diwali spike has become an annual flashpoint in Delhi. It reliably revives arguments over enforcement of firecracker rules and the failure to curb crop burning in neighbouring states.

Police across northern India had stepped up security for the festival, including checking operations in Jammu and Kashmir's Ramban district. In Delhi, the focus by nightfall had shifted to the haze settling over the city.