Dramatic footage also showed dark grey smoke billowing out from the windows.
The fire started at 10.30am in an office on the third floor of the grand building, which has seven floors. Some 100 firefighters were called to the scene.
It is as yet unclear what caused the fire and there are no reported deaths or injuries.
French police are asking people to avoid the area and the nearby Place de L’Opera has been shut along with the metro station.
A spokesperson for the fire service said at noon on Saturday that the fire was not yet completely under control and that they were working to stop it spreading. They added that it could take four hours to get rid of the blaze.
They said the risk of the fire spreading “remains real for either the adjacent premises or the upper floors”.
The Boulevard des Capucines is one of the “Grands Boulevards” in Paris. Its name comes from a convent of Capuchin nuns whose garden was on the south side of the boulevard before the French Revolution.
The Boulevard was also painted by Impressionist artist Claude Money in 1873.
Paris Police posted photos on Twitter of their staff getting ready to tackle the fire, with the caption: “The Chief of Staff of the Prefect of Police went to the PompiersParis (the firebrigade) and police officers mobilized following the Boulevard des Capucines fire.”
The fire service told people to “avoid the area and do not unnecessarily call the emergency lines”. Adding: “Residents, be careful.”
At least three fire engines were seen parked by the building. Crews wearing breathing apparatus used ladders to reach the flames and used high pressure hoses to put out the flames.
A spokesperson for the fire brigade, Florian Lointier, said: “There are no victims. When the firefighters arrived nobody made themselves known to them, be it in the offices or the dwellings.”