(CNN)Authorities in Johannesburg are scrambling to gain control of the city's cyber networks from hackers who are demanding payment in bitcoins.
Johannesburg city council member Funzela Ngobeni said the hackers gained access to the city's computer systems last Thursday and gave them until Monday to make the payment of four bitcoins, which is equivalent to 500,000 rands, according to the council.
The officials are refusing to pay and the ultimatum date has now elapsed.
"The city will not concede to their demands for bitcoins, and we are confident that we will be able to restore systems to full functionality," Ngobeni said in a statement.
Ngobeni called the breach an "attack on the people of the city" and said the hackers had targeted a period when residents were making monthly payments for utilities and also when the council pays its vendors.
Johannesburg City spokesman Nthatisi Modingoane said the hackers had sent messages to employees laptops asking for bitcoin payment, and authorities were forced to shut down the systems to avoid a further breach.
"They got messages from these people asking for bitcoin payment, and we just had to shut down all the systems," Modingoane told CNN.
The cyber breach has affected the city's online electronic platforms, including its billing and payment systems, he said.
He said a group of local IT and international forensic experts have been able to get 80 percent of its systems back online.
Residents in Johannesburg, a city of more than five million people, have been advised to pay cash in person at banks and use other third parties payment platforms for bill payments until the issue is resolved.
A ransomware attack on one of the city's electricity distribution company also left many residents without power in July.