A fire in an electricity sub-station started by vandals cut supplies to 27,000 homes in Birmingham.
Burning tyres were thrown into a duct carrying high voltage cables
The final 2,000 homes had electricity supplies restored on Monday evening, and officials expect to reconnect the final businesses on Tuesday morning.
Burning tyres were thrown into the sub-station in Saltley, starting the fire just before midnight on Sunday. Flames leapt 30ft (9m) into the air.
It affected Erdington, Ward End, Bordesley Green, Aston and Hodge Hill.
By 1000 BST on Monday, 8,000 homes remained without power, and that was down to 2,000 by late afternoon.
A spokeswoman for Central Networks, which supplies the area, said: "We ask customers to bear with us as we are doing all we can to resume supplies as soon as possible."
She added: "The people who did this were playing with their lives."
Electricity supplies at Heartlands Hospital were automatically switched to another source so that no patients were affected.
A hospital spokeswoman said that efforts had been made to keep demand to a minimum by using fewer computers and other electrical appliances so as to reduce pressure on supplies.
Parts of the motorway lighting network were also plunged into darkness.
A police spokesman said: "What appears to have happened is that someone has made a very determined effort to set fire to the substation, which has taken out the power supply.
"It's had far reaching implications for sections of north Birmingham."
A spokesman for West Midlands Fire Service said: "It's been extremely dangerous for crews here because of the high voltage and we've had flames 30ft in the air with smoke billowing across the whole of the area."