Men believed to be members of the banned Revolutionary People's Liberation Party-Front (DHKP-C) posted pictures on social media of a gun being held to Mehmet Selim Kiraz's head, threatening to shoot him unless authorities meet their demands.
The group's flag could be seen in the background as Mr Kiraz was held gagged and with his hands tied in front of him.
According to a statement released on halkinsesi.tv, the militants gave authorities until 3.36 pm local time (1.36pm BST) to meet their demands - three hours after they stormed the court building.
Speaking to the Hürriyet Daily News, Deputy Chief Prosecutor Vedat Yiğit said negotiations between militants and police "continue via a mediator that they picked".
"Our negotiators and Umit Kocasakal, the head of the Istanbul Bar Association, are talking to the militants," Istanbul Police Chief Selami Altunok said.
"We are trying to resolve the issue without anyone being hurt."
Special forces outside the İstanbul courthouse where prosecutor is held hostage by armed men. Photo via AP pic.twitter.com/8Dzo5bsRwd
— Abdullah Ayasun (@abyasun) March 31, 2015















Comment: This 'Marxist-Leninist People's Liberation Party/Front' (DHKP-C) has been accused of being one of the many faces of the Ergenekon network in Turkey, which is effectively NATO's stay-behind deep state Gladio structure in Turkey. The accusations are not without foundation... Like all such networks, it's not so much that their aim is to overthrow the governments of NATO member-states, but rather to ensure that their governments toe the party line (ie, Washington's line).