Explosions were heard near the Old Bailey on Wednesday morning before smoke poured out of buildings nearby.
There are reported to be power outages in the area and around 25 firefighters are on the scene.
Firefighters said they were responding to an incident at an electrical substation near the rear of the court.
Footage showed staff being evacuated from the court building shortly after the fire alarm went off at around 10.40am.
Footage showed black, acrid smoke billowing out onto the street by the court, known formally as the Central Criminal Court.
The London Fire Brigade said in a statement: “Four fire engines and around 25 firefighters are responding to an incident involving an electrical substation on Warwick Lane, near the rear of the Old Bailey.
“Traffic in the area is likely to be impacted as we respond. The Brigade received the first of six calls reporting the incident at 1044. Firefighters from Dowgate, Soho and Dockhead Fire Station are at the scene. The cause of the fire is not yet known.”
The court is the UK’s most high-profile criminal court. All proceedings at the Old Bailey are currently postponed as firefighters respond to the scene.
Among those taking place on Wednesday is the ongoing trial of Mark Gordon and Constance Marten, who are charged with manslaughter, concealing the birth of a child, and perverting the course of justice.
Also currently being heard at the court is the inquest into the deaths of three men who were murdered in a Reading terrorist attack, and the trial of Sandip Patel, who is accused of murdering prostitute Marina Koppel at her home in Marylebone in 1994.
The Old Bailey’s top judge, Recorder of London Mark Lucraft KC, has told reporters he heard the explosion and saw smoke.
He said it is “too early to say” when the courthouse might re-open.
Barristers decked out in traditional wigs and gowns have gathered on the pavement opposite the traditional courthouse following the evacuation.
After that incident, members of the City of London Corporation accepted there was a backlog of maintenance work at the courthouse which had been delayed for years. More than £7m has been put aside for upgrade work.
