Police and fire officials said the toll may rise, as firefighters struggled to enter the building of Omshakti fireworks factory.
Thirty people have died, police and fire officials at the spot said.
"These are times ahead of Deepavali. So activities go on at breakneck speed in the cracker units," Virudhunagar District Superintendent of Police Najmul Hoda said.
A factory official said 300 persons were at work at the unit at the time of the incident.
In October 2009, 32 people, most of them Deepavali shoppers, were killed at a cracker godown fire in Pallipattu in Tiruvallur district.