Thieves are being warned they are risking their lives and those of others by stealing street light cable.
More than £150,000 worth of cable has been stolen for its scrap value around Pontypridd, Rhondda and the Cynon Valley in the past year.
Live wires have been left hanging and roads plunged into darkness.
Police and councillors fear somebody will be killed and are appealing for residents to report suspicious activity.
The price of copper has risen substantially leading to a rise in thefts.
"It will come to a point where somebody will really get hurt," said Councillor Andrew Morgan, cabinet member for transport.
"When they're cutting these cables, they are leaving live cables hanging down.
"We're urging people that if you see anything suspicious at all ring the police or council and report it."
Thieves are also targeting underground cables.
One such incident took place on a designated "safe route" to a school at Glenboi near Mountain Ash in the Cynon Valley.
"They had dug down and cut through live underground cables which knocked out street lighting in homes," he said.
"It cost £14,000 to replace the underground cables."
Mr Morgan said the scrap value was just a small percentage of what it cost to replace the cables.
South Wales Police said not only were people putting themselves at risk by stealing copper wiring, they were also causing disruption to power and telephone supplies.
This could have serious consequences for the public and emergency services, officers point out.
Supt Simon Clarke said: "Cutting into any wiring, especially where the mains services are connected, is extremely dangerous.
"People who carry out this type of crime expose themselves and others to possible serious injury, or even worse."
In the past 12 months, thieves in RCT have also stolen £20,000 worth of manhole covers and litter bins to the value of £10,000.
Councils around Wales have seen an increase in metal thefts, with even statues being stolen.
Network Rail and police are also stepping up efforts to catch rail metal thieves, claiming they are costing the railways around £1m a year.
Anyone with information about cable thefts in RCT is asked to contact South Wales Police on 101, Crimestoppers anonymously on 0800 555 111 or the council on 01443 494700.