Police said the workers first took out a procession before coming to the mine and raising slogans against the management.
Regular employees of NLC are on strike since July 20 demanding wage revision.
The third round of talks, convened by Chief Labour Commissioner, was held in New Delhi on July 30 but no headway could be made with the unions and NLC management sticking to their respective positions.
Unions are insisting for 24 per cent increase while the management offered only 10 per cent.
NLC thermal units generated 2370 MW today against the total capacity of 2990 MW, he said.