Johannesburg - Numsa members working at the Medupi power station in Lephalale, Limpopo, marched against Eskom demanding better working conditions, the union said on Wednesday.
"We call on Eskom to respond to the demands of workers," the union's collective bargaining head Steve Nhlapo said in a statement.
The National Union of Metalworkers of SA's Medupi members wanted retrenchments of workers to be halted with immediate effect and for completion bonuses to be given to all workers - not only to senior managers.
They also called for an end to workers being reportedly evicted from their subsidised accommodation based on their race.
"The failure by Eskom to respond to these genuine and legitimate demands of workers will force the union to escalate its organisational muscle and power until these demands are met," Nhlapo said.
He said if the demands were not met the union would be forced to bring in workers from the Kusile power station in Mpumalanga.
"Numsa is convinced that the workers' reasonable demands can be resolved by Eskom, without any undue disruption of work."
He said Eskom closed the power station "as a direct response" to quell workers' action.
Eskom spokesperson Khulu Phasiwe confirmed that operations had been closed at certain parts of the power station and that some contractors had sent their non-striking workers home for safety reasons, while others continued to work.
"Our colleagues there are currently meeting with them [Numsa] so that we formally know what their grievances are and take it from there," he said.