Eleven people were taken to safety as firefighters battled a blaze in London's historic legal district.
About 150 firefighters were called to the Law Society's office in Holborn's Chancery Lane at 22:40 GMT on Saturday.
A junior lawyers division dinner had been taking place. About 28 people left the building before emergency services arrived and no-one was injured.
Chief executive Paul Tennant said he was "extremely upset" it had happened to a "wonderful building".
London Fire Brigade said much of the roof and part of the fifth, fourth and third floors were alight.
The fire was brought under control and fire crews will remain at the scene to minimise damage to the building.
"First of all I wanted to express my gratitude to the fire service and my relief that nobody has been hurt," said Mr Tennant.
"I also want to express my sympathy to the residents in the Chancery Lane area whom I understand may have had to evacuate their homes.
"It is too early to comment on the cause of the fire or the extent of the damage but clearly we are extremely upset that this has happened to this wonderful and historic building."
City of London Police also advised that a number of road closures were in place around Chancery Lane and has urged people to avoid the area if possible.
The cause of the fire is not yet known.
Holborn and the surrounding area has been associated with the legal profession since mediaeval times.