At least 12 persons were killed and over 20 injured when the roof in a section of a cement factory in Dalmiapuram near Tiruchirapalli caved in this afternoon, police said.
Twelve bodies were recovered, police said, adding that the number of casualties is likely to go up to at least 20.
The injured have been rushed to various hospitals near the factory.
Police and fire service personnel and the factory's security staff are engaged in the rescue operations.
The mishap occurred at around 1315 IST in the stacker-reclaimer division of the 70-year old Dalmia Cement Bharath Ltd, around 45 km from Tiruchirapalli.
Police said the labourers were trapped inside when the freshly laid roof concreting, part of the expanded division, caved in.
A huge pillar, which was supporting the roof, had also fallen and some persons are feared trapped beneath it.
Efforts are being made to remove the pillar, police said.