An electrical fire under the pavement in central London led to some 5,000 people being evacuated from nearby buildings and the cancellation of a number of West End shows.
Fire crews were called after smoke was seen coming out of an inspection cover on Kingsway, in Holborn, at 12:30 BST.
The blaze is now under control, said London Fire Brigade.
It said crews would be working "through the night" to put the fire out and stop it spreading to buildings in the area.
Earlier, onlookers reported struggling to breathe and "chaos" in and around Holborn.
Shows at the Lyceum, Duchess and Aldwych theatres were cancelled, London Theatre Direct said.
A father and his young son who had to leave the Royal Drury Lane theatre midway through a matinee performance of Charlie and the Chocolate Factory said: "There was a big blackout and we were sat in the dark for half an hour and they decided to call it a day, so we've missed out on seeing Willy Wonka now - it's chaos."
Wednesday's evening performance was cancelled.
A restaurant owner in the area said he would not be able to open for the evening which would cost him about £10,000 in lost business.
Earlier, judges, lawyers and other staff at The Royal Courts of Justice in The Strand, as well as buildings along Rose Street and Garrick Street and the market building in Covent Garden were evacuated.
UK Power Networks said 1,900 customers in the area were left without electricity and it had so far restored power to 1,300.
A spokesperson said: "Crews are currently working hard to excavate and isolate the damaged parts of the network, then reroute power to the remaining 600 customers overnight."
The company said it could be difficult to excavate in central London but as long as the work went well the network should not need to switch off supplies on Thursday morning when the demand increases.
Earlier, BBC London 94.9's Anna O'Neill spoke to Gary Squires from London Fire Brigade at the scene of the fire.
He said the fire started in a large service duct which carries electrical cables and gas and water pipes beneath the street.
"We have some very high-powered cables down there which is causing a hazard to our crews," he said.
Mr Squires said: "We can see flames coming out of the pavement, which looks quite dramatic, but that is the gas pipe - it has now ruptured and that has now ignited and the gas is burning off and that, if anything, makes it safer for us as the gas is being burnt off."
He said lots of people had "self evacuated" from nearby buildings and it was only the ones on Kingsway that had been evacuated by the authorities.
A National Grid spokesman said engineers were called to the scene as a precautionary measure but no problems had been found.
Eyewitness Nicola Foxfield, 29, said she could "smell burning in the air" and saw thick, brown smoke coming off the road.
"It's chaos and fire engines are coming from every direction."
Another eyewitness said: "I was close enough to see thick, black smoke puffing out of the drain grills and I struggled to breathe."
The fire caused 10 bus routes to be diverted, according to Transport for London (TfL), and Holborn station is closed. TfL said Tube tickets were being accepted on local buses in zone one.
Kingsway is closed between Holborn and Aldwych, the Strand Underpass and Waterloo Bridge northbound, causing serious delays.