The fault in a power transmission line has affected "in excess of 100,000 properties" stretching from Inverness to the far north of mainland Scotland and the Western Isles, Orkney, Shetland and even the Isle of Skye.
It is thought to be the worst power blackouts in decades in Scotland.
Pubs and hotels in Orkney and Stornoway reported how they were left to rely on candles for about an hour after the incident.
Internet connections were also downed by the blackouts as word spread of the cuts.
Inverness was plunged into darkness for about 30 minutes and rail services in and out of the city and to the far north of the country were also delayed. Skye also had no power for about 90 minutes.
The problem centres on a major problem affecting a energy supplier SSE Power Distribution.
The company says it has suffered a major network transmission fault and its engineers are battling through the night to restore supplies.
A spokesman apologised and said they had "no idea" when supplies would be returned to normal.
One bar worker in Stornoway said: "Although the power supplies were only out for about 30 minutes here, we had people sitting around the pub in darkness. It was the same across the town."
A member of staff at the Kirkwall Hotel said: "The lights were out for just over an hour here and some of the guests were coming down and wondering when it was coming back on.
"We had electric candles that we were able to power up in the restaurant. It's very unusual to get a power cut for so long."
A voicemail on the customer services line for SSE said it was "experiencing a major transmission network fault problem affecting in excess 100,000 properties".
The voicemail added: "Our engineers have been mobilised to primary locations across the country and have begun the process of restoring supplies.
"Due to the nature and scale of the fault we are not able to confirm details of the restoration of supplies."
The statement added that it expected most supplies would have been reconnected by midnight, but this could not be guaranteed.
An SSE spokesman said: "We are aware that there is an issue affecting the network in parts of Northern Scotland and are working to identify the fault and restore power as quickly as possible. We would encourage any customer affected by a loss of supply to call us on 0800 300 999.
"We would like to apologise for any inconvenience caused and thank customers for their patience while we work to rectify the problem."