The town hall in the French city of Limoges was "largely lost" to a fire set by masked protesters on Saturday, the top regional official told AFP.
"The roof has collapsed onto lower floors, except the west wing, and two-thirds of the exterior has already disappeared," the local prefect, Maurice Barate, said.
He called the extensive damage to the 143-year-old building a "catastrophe for heritage".
"A wild horde" had "deliberately" set fire to the town hall, Barate told TV news channel LCI on Sunday, adding that the fire had now been contained.
The arson occurred during unrest that gripped France on Saturday during nationwide demonstrations over the cost of living.
A group of masked protesters who had torched a police car outside the town hall were seen breaking windows and entering the building to set a fire, according to witnesses.
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Firefighters were deployed late Saturday to try to contain the blaze.
As hundreds of people watched the blaze, a municipal employee was seen in tears comforted by a local councillor.
Several shops in the centre of Limoges were also looted.