Sikonathi Mantshantsha said groups of people, some wearing union outfits, were protesting outside the Duvha, Hendrina, Matla, Arnot, Medupi and Matimba coal plants.
"Some of these protesters have blocked roads leading to some of the power stations, which hampers the movement of people and goods into or out of the facilities," he said in a statement.
"While some incidents of intimidation have been reported, the protesters are largely peaceful at this stage."
Police have been deployed.
On Wednesday, Eskom said salary negotiations with unions including the National Union of Metalworkers of South Africa had reached a deadlock after multiple rounds of talks.
Eskom has struggled to meet electricity demand in Africa's most industrialised economy for over a decade, constraining growth and deterring investment.
(Reporting by Anait Miridzhanian; Editing by Alexander Winning)