It was not immediately clear whether the incident was linked to Friday’s abortive military coup in which more than 200 people were killed.
Turkey remains in a state of high tension, though the government says it has the situation fully under control.
NTV reported that the assailant had entered the office of deputy mayor Cemil Candas and then gun shots were heard.
Turkey’s main opposition Republican People’s Party (CHP) runs the prosperous Sisli district. Along with other opposition parties the CHP has condemned the attempted coup.