(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.

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