Flames shot 15ft into the air when electricity cables beneath a manhole cover blew up yesterday, just after 2pm.
Most of King Street and surrounding roads lost power, with customers and workers having to be evacuated as shops and businesses were plunged into darkness.
Police and fire crews cordoned off the roundabout and pavements around Harton Staithes at the top of Mill Dam, while officers waited for safety experts and engineers to arrive.
Power was restored by about 4pm and an investigation was launched to find out the cause of the blast.

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SOUTH Shields town centre ground to a halt after an explosion beneath a manhole cover knocked out power to 261 buildings.
A witness reported flames shooting up to 15ft from the path as the metal cover was launched into the air yesterday.
Most of King Street in the town centre came to a standstill just after 2pm as power supply and telephones were cut off, and shop bosses were forced to evacuate customers.
Mum-of-one Ameri Byrne, 31, from South Shields, was driving past the manhole cover at the time of the explosion.
She said: "I was driving along when the cover flew off, there was a loud bang and huge flames shooting from the ground – I thought it was a bomb.
"I had to swerve away from it because I didn't know what was happening. Thank God nobody was hurt. I'm so shocked."
Fire crews rushed to the scene and police closed off Ferry Street from the top of Mill Dam, diverting traffic along Coronation Street and away from the area.
Meanwhile, as police searched for the missing manhole cover – which was later found back down the scorched hole – community support officers were drafted in to stop buses heading up Chapter Row.
A member of the rapid response team from electricity suppliers CE Electrics was at the scene, a gas official also attended as a precaution, and after almost two hours of working on damaged cables, electricity was reconnected through a second cable.
Another team was then sent out to find the cause of the problem and repair the damage, and it was discovered that a link box beneath the ground, where two cables meet, had malfunctioned.
Metro services in to the town centre suffered minor problems with the ticket machines at South Shields cutting out, but the main supply to the station was quickly redirected via another source.
A spokesman for CE Electrics said an investigation is ongoing: "Most places are connected by two cables so if something happens to one, premises with no electricity can be connected through the remaining cable.
Problem
"There are a number of reasons why a problem with cables can occur, usually there has been some sort of excavation work somewhere and they are unaware they have cut the cable and when rain gets in, it causes a short circuit."
The spokesman added that a fault was found in a high voltage cable with possible damage to a near-by low voltage cable.
He said this will affect street lighting in the immediate area.
However, while it is thought this will take a few days to repair, engineers are working as quickly as they can to correct the problem.
A spokesman for South Shields Fire Brigade said: "We received a report of an explosion in the roadway so we took precautionary measures until the electricity board got there.
"We believe the explosion was caused by a problem with an underground cable and the force of it blew off the manhole cover."

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SHOPPERS and store workers were left bemused as most of South Shields town centre was plunged into darkness.
The blackout left hundreds of staff and customers with nothing to do but head home, although Asda and the shops between Card Factory and Bright Homes in King Street were unaffected.
Undeterred by the power cut, newsagent Douglas Hepburn, 43, still sold copies of the Gazette from the doorway of his shop, Mayfayre News, in King Street.
He said: "The power cut was a massive knock to our takings, but I salvaged some money by selling the newspapers in the doorway."
Tracy Hancock, a sales assistant at jewellers Herbert Brown in King Street, said some staff had to stay behind for security reasons, as no alarms were active and the shutters were up.
She added: "There was no heating on and it was getting cold. We were worried we could have been stuck inside until late into the night."
Steve Jones, assistant manager at T-Mobile in King Street, said: "The shutter is on a motor so we couldn't go home."
Dave Anderson, manager of menswear store Burton in King Street, said: "The lights just went out and all the power went off. It was pandemonium."
Diners at McDonald's in King Street continued eating in the dark.
Manager Steven Latimer said: "We're open until 11pm, so it was just a case of keeping busy while we waited for the power to be restored.
"We waited until everyone had left before we closed."
The power cut also hit fashion stores in Waterloo Square and it also thought to have affected businesses and factories in the Mill Dam area.
River Island manager Helen Brown said: "Around 2pm is our busiest time of day, so closing early was a huge blow to the day's business."
Staff at Argos were sent home as they thought the power would not return.
Kenneth Hope, 76, from East Boldon, who was returning from a shopping trip in Newcastle with wife Jessie, 82, said: "It was like the war."

GAZETTE REPORTING TEAM
gazette.news@northeast-press.co.uk