City Power warned customers on Friday they could not report faults via the city's call centre due to a computer network security breach by a group seeking a bitcoin ransom.

The city reported a breach of its network late on the night of October 24 and shut down its website e-services.

“They have isolated their network and applications,” said City Power spokesperson Isaac Mangena.

“This affects City Power in that our customers will not be able to log calls on the city of Johannesburg call centre.”

The customer billing system was also affected. Customers were urged to use the Citypower.mobi app to log calls instead.

Here's what you need to know about the cyber attack.