Several parts of Pretoria have been left without electricity amid an "illegal" strike by City of Tshwane staff.

At least four regions have been affected during three days of outages.

The affected areas included Kwaggasrand, Westpark, Saulsville, Danville, Laudium, Valhalla, Erasmia, Rua Vista, Thatchfield, Reeds, Olievenhoutbosch and Zithobeni.

The City said in a statement that technicians were in the process of restoring power to some areas that have experienced prolonged outages. However, the City said staff faced threats and intimidation by striking employees.

The reasons for the strike were unclear.

"Tshwane has experienced a series of outages affecting at least four regions due to the strike, and the City administration has now enlisted the services of law enforcement, who are escorting teams to attend to service interruptions in a [bid] to clear the backlog," the statement read.

The strike started on Tuesday.

There were also widescale disruptions of waste collection services due to the strike.

The City said it established a joint operations centre on Thursday to "restore order and put an end to service delivery interruptions". This includes enlisting the help of the police and Tshwane Metropolitan Police Department, as well as private security companies.

"We have deployed law enforcement teams to monitor hotspot areas and ensure high police visibility," said MMC for Community Safety Grandi Theunissen.

"The City has also put in place surveillance measures to identify the instigators behind the illegal strike who are engaged in criminal activity, such as destroying property and intimidating staff," Theunissen added.