Firefighters from three stations were fighting the blaze on Tuesday morning.
“We don’t have any information that anyone is injured,” said police spokesman Björn Engström. The nearby Munksbron bridge was closed for car traffic.
Flames were still rising from the roof of the building at 8.45 am, according to news agency TT. Police were placing a cordon around the area to make space for firefighters. Several fire engines from three fire stations were attending the scene.
The affected building is a four-storey apartment block, 11 Munksbron. The fire started at the top of the building and spread to the roof. Svenska Dagbladet reports the emergency services saying that the fire could spread to neighbouring buildings.
The building next to the burning block, 4 Lilla Nygatan, is owned by Sweden’s parliament, the Riksdag, and is used to provide Stockholm accommodation for members of parliament. Around 60 MPs live in the building, including ex-prime minister Göran Persson. Emergency services say all have been evacuated.
The fire service has special procedures to deal with fires in Gamla Stan, owing both to the historical value of the buildings there, and the fact that they are old and more vulnerable to fire. The situation is also complicated by the areas narrow streets.