Two thousand people have been evacuated from their offices after a fire broke out among electrical cables under a pavement in Holborn.
Seventy firefighters are now working to prevent the spread of the fire after the cables caught on alight in Kingsway. Plumes of thick smoke can be seen engulfing nearby streets.
"We are working closely with the various electricity companies involved to identify the source and extent of the fire which is underground," assistant commissioner Peter Cowup from the London Fire Brigade said.
"As this is likely to be an electrical fire we have to ensure the power is off before we apply water to extinguish it.
"We are working with our partners and implementing well-rehearsed plans to prevent the fire from spreading and to protect surrounding properties."
People from the Lyceum Theatre, which has cancelled the 1 April showing of the Lion King, have been evacuated along with judges, lawyers and staff from the Royal Courts of Justice, situated on the Strand.
The Aldwych and Duchess theatres have also cancelled their performances of Beautiful and The Play That Goes Wrong due to the fire.
A 25m safety cordon is in place while the London Fire Brigade and police deal with the "major incident" and have closed Kingsway in both directions.
A spokesman for London Fire Brigade said: "The fire continues to still burn. There are a lot of electrical cables down there. But we cannot tackle it effectively until the power situation has been resolved."
The alarm was raised after smoke was seen coming out of an inspection cover onto the pavement.
Teams from electricity firm, UK Power Networks, which is at the scene, have warned there will be be temporary power cuts while the crews control the blaze.
A UK Power Networks spokesman said: "Power supplies in the area have been interrupted and as soon as we can gain access, we will work to restore electricity as quickly as is safely possible. We will also help the emergency services investigate the cause."
Motorists are also being advised to avoid the area and Holborn station is currently closed.