The Old Bailey was plunged into darkness and about 1,500 people were evacuated from the historic building after a loud bang was heard and black smoke billowed nearby.
Dozens of barristers in black gowns and wigs lined the street on Wednesday morning following the fire alert at the Central Criminal Court in central London.
Four fire engines and around 25 firefighters were called to the scene at the rear of the building.
The Recorder of London, Judge Mark Lucraft KC, was seen liaising with court staff and firefighters who had entered the Old Bailey and gathered around Warwick Lane.
Barristers and other court users were told by security officials that they would not be allowed to go back inside until after safety checks had been carried out.
London Fire Brigade said: “Firefighters are currently responding to reports of smoke issuing from a building in Warwick Lane.
“Four fire engines and around 25 firefighters are responding to an incident involving an electrical substation on Warwick Lane near the Old Bailey.
“Traffic in the area is likely to be impacted as a result.”
Defendants were seen being walked out of a side exit and into a waiting prison van.
They were escorted by uniformed City of London Police officers.
Temporary Chief Inspector Tom Fisher, of City of London Police, said: “There is no indication at this stage that this is anything other than an incident involving an electrical sub station.
“London Fire Brigade reported that around 1,500 people were evacuated from the Old Bailey and surrounding area.”
The Old Bailey remained closed on Wednesday afternoon, with barristers advised to wait to be escorted into the building to collect their belongings safely.
Hundreds of people, including jurors, defendants and lawyers, pass through the Old Bailey every day.
There are 18 courtrooms in operation, hearing major cases including murder and terrorism.
Among the high-profile trials abandoned for the day was that of wealthy aristocrat Constance Marten and her partner, Mark Gordon, who are accused of the manslaughter of their newborn baby daughter.