Ten fire engines and around 70 firefighters attended a fire in an underground utilities tunnel on Theobalds Road, by the junction of Southampton Row, in Holborn.
The fire is now under control, but crews will remain on scene throughout the afternoon. There were no reports of any injuries.
The Brigade’s Fire Investigation team will now begin its investigation into the cause.
Station Commander Paul Morgan, who was at the scene, said: "Six fire engines and around 40 firefighters were initially mobilised to the scene, where crews discovered a fire involving high-voltage electrical cables below street level.
"Due to the challenging and hazardous nature of the incident, an additional four fire engines were requested to attend, taking the total to 10 appliances. This was a prolonged operation given the complexity of tackling a fire within an underground electrical tunnel. Once the power was isolated, firefighters wearing breathing apparatus entered the tunnel to better tackle the fire.
"Working closely with partner agencies, firefighters safely brought the fire under control.
"The incident is still causing significant disruption in central London, with multiple road closures and a safety cordon currently in place. Several bus routes have been diverted, and the public are advised to continue to avoid the area where possible.
"Our Community Engagement team is on site providing support to local residents and businesses affected by the fire."
The Brigade's 999 Control officers took the first of 18 calls to the fire at 0941 and sent crews from Soho, Euston, Shoreditch, Islington, Lambeth and surrounding fire stations to the scene. The fire was under control by 1407.
Assistant Commissioner Pat Goulbourne said: “Over the past two days, London has experienced two significant fires that have caused disruption. On Tuesday, we declared a major incident following a warehouse fire in Southall, and today a separate fire has significantly impacted central London.
“I want to express my sincere thanks to all firefighters, officers, staff and our partners who have worked tirelessly in challenging conditions to protect the public and keep the city moving.
"We will continue to work closely with our partners and remain fully prepared to respond to any further incidents to keep London safe.”