The utility, which has struggled to meet power demand in Africa's most industrialised nation for over a decade, has been implementing rotational outages since the start of the week that are due to last until Sunday night.

Eskom said in a statement that the protests included incidents of intimidation of working employees and blockading of roads leading to power stations.

"Should these criminal acts of intimidation persist or spread, this would increase the risk of ... load-shedding at higher stages," Eskom said, using a term for electricity outages.

Eskom spokesperson Sikonathi Mantshantsha said on Thursday that groups of people, some wearing union outfits, were protesting outside six coal plants and the company said police had been deployed.

The negotiations with trade unions including the National Union of Metalworkers of South Africa broke down on Tuesday.

Eskom said on Friday that over the past 24 hours there had been protests at nine of its power stations and other operating facilities, by some of its employees who had embarked on unprotected strike action.

The company said it was exploring possible solutions to unlock the deadlocked wage talks, reminding protesters that electricity provision was considered an essential service in which industrial action was not permitted.

The loss-making utility, saddled with a huge debt pile approaching 400 billion rand ($25.12 billion), is trying to contain costs as part of turnaround efforts under Chief Executive Andre de Ruyter.

Reforming Eskom is a priority for President Cyril Ramaphosa's government, but efforts to improve power plant performance are yet to bear fruit.

Eskom is currently implementing "Stage 2" rotational cuts that require up to 2,000 megawatts (MW) to be shed from the national grid. The company has a total nominal capacity of 46,000 MW, but earlier this week more than a third of that capacity was offline due to breakdowns and planned maintenance.

($1 = 15.9266 rand)

(Reporting by Anait MiridzhanianEditing by Alexander Winning and Susan Fenton)