Another 15 people were wounded in the blast in Tamil Nadu state, which came days before the Hindu festival of Diwali, which many celebrate by letting off fireworks.
Twin fires kill at least 22 in India
"Eight workers were killed and 15 were injured. Some of them have critical burn injuries," local police inspector Vijaya Lakshmi told AFP.
Emergency workers evacuated patients from a nearby hospital fearing the fire could spread.
The town of Sivakasi where the blast occurred is home to some 700 small factories producing fireworks that are running at peak production ahead of Diwali.
Indian police arrest doctor and three others over deadly hospital fire
More than 35 people were killed in 2012 after one of the factories in the town caught fire leading to a massive explosion.
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ