Firefighters have been tackling a large fire at the old Crumlin Road courthouse in Belfast.
It started at about 02:43 BST on Monday with 25 firefighters, three appliances and an aerial appliance trying to dampen down the blaze.
A PSNI spokesperson said the fire was being treated as deliberate.
Designed by Sir Charles Lanyon in the 1840s, the courthouse is one of Northern Ireland's most distinctive buildings.
It closed in 1998, after nearly 150 years of use, but had already been damaged by previous fires.
North Belfast MP John Finucane said the building was one of the most historic and recognisable in the city.
"This deliberate attack on it is wrong and must be condemned," he said.
Alderman Brian Kingston said because of this he "feared that little of the original content will now be left".
He commended the firefighters who put an appliance behind the courthouse to protect houses in Lower Shankill estate.
The DUP politician added that the building should not be "left in the hands of private developers while it continues to deteriorate and to be damaged".
The Belfast Buildings Trust said it was "deeply saddened by yet another fire" inside the historic building.
Development Manager Shane Quinn said it was "hugely troubling," that it may have been started deliberately.
"The courthouse's physical history and the personal stories associated with it say so much about Belfast," he said.
He added that while the full extent of the damage needed to be assessed he was confident the building could be regenerated.
The listed building, which has planning permission to be converted to a hotel, was put on the market by Signature Living in 2019.
It is the second fire in the space of 24 hours in Belfast after a "significant" grass and gorse fire broke out in near the nature reserve at Belfast Harbour Estate.
Plumes of smoke were visible over the city on Sunday evening.
The Northern Ireland Fire and Rescue Service (NIFRS) said about 40 firefighters were, at one stage, tackling the blaze.
It was brought under control later on Sunday.