The blaze broke out during renovation work at the Masquerade nightclub situated on the ground floor and basement of a 16-storey residential building in the Gayrettepe district of the city.
It was extinguished and Davut Gul, the Governor of Istanbul, said an investigation was under way.
The victims are thought to have been involved in the renovation work.
At least one person was being treated in a hospital, the Istanbul governor’s office said in a statement.
Police detained five people for questioning, including managers of the club and one person in charge of the renovations, Justice Minister Yilmaz Tunc said.
Footage posted by the Istanbul Fire Department on X shows crews attending to the scene with several fire engines and ambulances surrounding the smoking entrance to the building.
Mayor of Istanbul, Ekrem Imamoglu, said on X: “May God have mercy on our citizens who lost their lives, and I wish a speedy recovery to our injured.”
Mr Imamoglu added authorities were inspecting the whole building to assess its safety.
