A road in central London remains closed as an electrical fire continues to burn under the pavement.
Some 5,000 people were evacuated from nearby buildings after smoke was seen coming out of an inspection cover in Kingsway, Holborn, on Wednesday.
London Fire Brigade (LFB) said the fire has been contained but has been "technically difficult" to tackle.
More than 20 firefighters and officers are still at the scene and local road closures are in place.
The cause of the fire is still unknown.
LFB Assistant Commissioner Peter Cowup said: "This has been a technically difficult incident to tackle.
"The reason that the fire is still burning is because the service tunnel is hard to reach.
"Although firefighters have been applying water through access points throughout the night, the complexity of the tunnel layout means that it will be some time until the fire is fully extinguished."
He added LFB, the Met Police and utility companies were making steady progress to try and resolve the situation.
'Not an overnight job'
Speaking to BBC London 94.9, Insp Neil Johnson, from the Met Police, said: "The fire is still live in the subway. The problem at the moment is there is a gas pipe underneath and we are all in agreement that it is ok and if we can keep it contained it will be fine.
"All we need to do is keep people out of the area and let the fire brigade and utilities do their job."
"I imagine this road will be closed a long time after this is finished because of damage the heat does to the road," Mr Johnson said.
"It will have to remain closed until a structural engineer examines it properly and either he or she says what work has to be done and that work is completed - this is not an overnight job."
On Thursday onlookers reported struggling to breathe and "chaos" in and around Holborn.
UK Power Networks said the number of customers currently affected by the power cuts stood at about 1,000 and it had restored power to about 2,000.
Apologising to customers Matt Rudling, from UK Power Networks, said: "The gas is still burning under there and until we can gain access to that particular area we won't understand what's caused it and what we can do."
"We want to try and restore [power to] all those remaining customers by the end of day today."
He apologised for the disruption and added that emergency generators were being used to supply power to the area.
UK Power Networks' engineers are also trying to connect some of the damaged cables to unaffected ones.
