NEW DELHI: An explosion at an illegal firecracker factory in eastern India killed seven workers and injured nine others in the hours before Thursday's Diwali festival, officials said.

Firework use hits a peak across India during the Hindu festival but New Delhi authorities have tried to restrict sales to tackle mounting pollution.

The explosion late Wednesday completely destroyed the makeshift structure after fire touched off the gunpowder and chemical stocks used to make the fireworks in Balasore district of Odisha state, said district magistrate Pramod Kumar Das.

He told AFP several of the injured workers are in a critical condition after the "huge" explosion.

Diwali, the festival of lights, is traditionally celebrated by lighting lamps but has metamorphosed into a grand show of fireworks, sparking pollution and controversy.

Explosions often occur in the thousands of illegal backyard and underground workshops that spring up during the festive season.

Last month, nine people were killed in neighbouring Jharkhand state after their workshop was gutted by fire.

India's firecracker industry, worth nearly one billion dollars a year, is the second largest in the world after China.

The country's Supreme Court this month temporarily banned the sale of firecrackers in New Delhi because of the air pollution threat.

The ruling came after the capital last year suffered its worst air pollution in nearly two decades, which experts blamed on Diwali fireworks and stubble-burning in farming regions around the city.

Police have arrested more than two dozen people in New Delhi over the illegal sale of firecrackers since the October 9 court order and have seized more than one tonne of firecrackers.