It was an official who discovered the cyber attack after the hackers left a message on the city's login system.
“Your servers and data have been hacked. We have dozens of back doors to your city. We have control over everything in your city. We have also compromised all passwords and sensitive data, such as financial and population information, ”said the message, reported the technology news site Zdnet.
After the discovery, several of the city's IT systems were shut down to prevent the attack from spreading further.
Will not pay
The hacker group is demanding that the city pay a ransom of 4 bitcoin, about SEK 290,000, at the latest Monday - but according to a press release from the city, the requirement will not be met.
"I can confirm that the city will not agree to their demands, and we are confident that we will be able to restore our systems to full functionality," a spokesperson wrote in a statement on Johannesburg's official Twitter account.
Has happened before
This is not the first time that Johannesburg's infrastructure has been cut by hackers. In July, one of the city's largest electricity suppliers suffered a cyber attack, even where the perpetrator demanded a ransom to restore the vulnerable IT system. The BBC reports.