Johannesburg - Eskom was planning countrywide power cuts over the weekend to build up reserves, the parastatal said on Thursday.
"The electricity system will remain constrained tomorrow [Friday] and the risk of load shedding remains high," it said in a statement.
"Eskom has also taken a decision to load shed over the weekend in order to build up reserves for the week ahead."
The utility announced power cuts countrywide between 16:00 and 22:00 on Thursday.
Diesel supplies
"The power system is exceptionally constrained today [on Thursday] due to unforeseen technical problems at power stations, depleted water reserves and logistical issues relating to diesel supplies at our peaking power stations.
"As a result Eskom has implemented stage two load shedding."
City Power said the rolling blackouts would affect several suburbs in Johannesburg and it advised customers to check its Twitter account for regular updates. A load shedding schedule was available on its website.
The City of Tshwane and Cape Town warned residents of the blackouts on Twitter.