The attack took place at the well-known Reina nightclub in the Ortakoy neighborhood, in Istanbul’s Besiktas district.
There were two attackers involved, according to NTV, but conflicting reports also described a lone gunman.
The gunmen were dressed in Santa Claus outfits, wielding assault rifles, Turkish media said.
The attackers opened fire on clubgoers, injuring at least 20-30 and killing two, according to NTV. One of the gunmen has reportedly hidden inside the club, while the whereabouts of the second one were not immediately clear.
The number of casualties may rise, as it is estimated that between 700 and 800 people could have been in the club at the time of the attack, Mynet Haber reports. Emergency crews have been evacuating injured people from the building as police search the area.