More than 2,000 people were evacuated from buildings in central London this afternoon as toxic smoke filled the streets.
The Metropolitan Police declared a "major incident" after blocks housing businesses and part of the London School of Economics were cleared by the emergency services.
As the smoke spread, Holborn Tube station was closed and the Royal Courts of Justice also had to be evacuated along with some nearby West End theatres, meaning shows were at risk of being cancelled.
Underground fire in Holborn, central London. pic.twitter.com/6Az4djVXF9— Rickin Majithia (@rick_maj) April 1, 2015
After battling the fire for around nine hours, the London Fire Brigade has tweeted to say that the blaze is under control, but around 21 firefighters will remain at the scene overnight to monitor the situation.
Benedict Cumberbatch's appearance at Letters Live at the Freemasons' Hall was among the events called off.
Firefighters said the fire started among power cables beneath the pavement in Holborn and they could not put it out until the current was switched off.
The blaze was believed to have originated in a service duct that carries gas, electric and water through the capital.
The London Fire Brigade worked with utility companies to find the source of the fire, which appeared to be worsening hours after it began.
Footage from a Sky News helicopter captured flames bursting out of one manhole in a closed off road after a gas main ruptured underground.
Photos from the scene showed the air filled with dark smoke that appeared to be streaming from potholes and underground vents.
People were seen pouring out of offices, restaurants, cafes and shops while roads were cordoned off by the emergency services.
Onlookers described "crazy scenes" and "madness" as smoke rose so high in the air that it could be seen from the Shard skyscraper.
"Holborn is chaos. All the manholes on Kingsway are smoking," Jordan Hatch wrote on Twitter.
There have been no reports of any injuries but a witness told the BBC the air was difficult to breathe.
“I had just cycled over Waterloo Bridge and as I approached Kingsway I saw a big crowd of people and smoke billowing towards me," he said.
"The fire crews and police officers were beginning to cordon off the area. I was close enough to see thick black smoke puffing out of the drain grills and I struggled to breathe.”
Several buildings also suffered power cuts due to the blaze, forcing some companies to send staff home, and Holborn Tube station was closed.
The alarm was raised at around 12.40pm after smoke was seen coming out of an inspection cover on the pavement in Kingsway.
A 25 metre safety cordon resulted in around 2,000 people being evacuated from nearby buildings, including the Civil Aviation Authority’s headquarters.
At one point, around 70 firefighters and 10 engines were at the scene, as police advisd drivers to avoid the area due to road closures.
Transport for London said Holborn Underground station would remain closed until the power supply was fixed and warned that closures on Kingsway and in the Strand Tunnel were causing queues back to Trafalgar Square.
Performances of West End shows including Mamma Mia at the Novello,The Lion King at the Lyceum, Beautiful at the Aldwych, Play That Goes Wrong at the Duchess, Charlie and the Choloate Factory at Drury Lane, Woman in Black at the Fortune, and Gypsy at the Savoy were cancelled according to London Theatre Direct.