City Power, responsible for powering South Africa's financial capital Johannesburg, said on Thursday it had been hit by a ransomware virus that had encrypted all of its databases, applications and network.
The municipal body, owned by the City of Johannesburg, said its website was down and that customers would struggle to access a number of services, in particular buying electricity and uploading invoices, and it could affect its ability to respond to some outages.
"The virus has attacked City Power database and other software, impacting most of our applications and networks," City Power said in a tweet.
"We apologise to our customers in the @CityofJoburgZA," another tweet said.
Johannesburg residents calling a local radio station complained they had been left without power as a result of the ransomware, a type of virus that blocks access to computer systems or files until a demand is met, usually for money.
City Power said it was currently working to restore all the impacted applications.