The Revolutionary People`s Liberation Party-Front (DHKP-C) published a picture of the prosecutor with a gun to his head.
Shooting could be heard inside the court building, Sabah newspaper said.
A local TV showed special forces officers entering the building and courthouse officials being escorted out.
Turkish special forces entered the courthouse in Istanbul where a prosecutor, Mehmet Selim Kiraz, is being held hostage by an armed far-left group, after the building was surrounded.
The Revolutionary People`s Liberation Party-Front has reportedly announced five demands.
The one of them is a negotiator. The rest four demands have been not disclosed yet.
The group promised to kill the hostage if the building was stormed.
The Turkish prime minister has the situation under control despite the meeting with the ministers over the recent blackout across the country.
Meanwhile, the father of Berkin Elvan has demanded not to kill the prosecutor: "My son is dead, I don`t want anyone else to die. Let him go!"
Berkin Elvan was hit on the head by a police officer in June 2013, during mass anti-government rally in Istanbul. He died in March 2014, after 269 days in a coma. The investigation into the Berkin Elvan case is ongoing.
The DHKP-C is a radical leftist group, founded in 1978 as Revolutionary Left, and then re-branded in 1994. The organization is considered a terrorist group in Turkey.
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