Police arrested the factory owner over the explosion which happened late Wednesday at a village in West Bengal about 130 kilometres from the state capital Kolkata, Inspector General Anuj Sharma said.
Fireworks are a major feature of religious and other celebrations held throughout the year in India.
Fatal accidents are common in often makeshift and illegal factories or in people's homes where sparklers, firecrackers, bottle rockets and other fireworks are put together.
"Eleven people were killed and six injured in (an) explosion at an illegal firecracker factory at Pingla" village, Sharma told AFP.
"One person was arrested in connection with the incident," he said.
An explosion at a firecracker factory in southern Andhra Pradesh state in September left at least eight people dead.
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