
Local police chief N. Shreenatha told AFP that "20 people are confirmed dead" after the incident at a licensed factory in Tamil Nadu state's Virudhunagar district.
Rescuers were still operating at the site, he said, adding that the cause of the the blaze was unknown.
Industrial accidents are common in India, often due to poor adherence to safety regulations and weak enforcement.
Prime Minister Narendra Modi, in a social media post, extended his "condolences to those who have lost their loved ones" in the "deeply distressing" incident.
Tamil Nadu Chief Minister M.K. Stalin said the deaths were "tragic", expressing his "immense sorrow" in a post on X.
An explosion at a power plant in central India this week killed more than 20 people.
Last month, another fire at a fireworks factory in western India killed 17 people, reports BSS.