The armed man is holding doctors hostage at the Cerrahpasa Tip Fakultesi Hospital
The man is believed to have come from the psychiatric unit at the Cerrahpasa Tip Fakultesi Hospital, the Istanbul University Hospital.
The news comes as Turks are on edge a month after Islamic State (ISIS) jihadis attacked the popular upmarket Reina nightclub on the shores of the Bosphorus in central Istanbul.
A total of 39 people were killed and at least 70 were injured in the incident after gunman Abdulkadir Masharipov entered the nightclub and started shooting.
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