Armed men have taken hostage the Turkish prosecutor heading the inquiry into the death of a boy during anti-government protests in 2013.
Dramatic images have emerged on social media of a gun being held to the head of Mehmet Selim Kiraz at a court house in Istanbul.
A banned Marxist revolutionary group is said to be behind the incident.
A statement posted online said the prosecutor would be killed if their demands were not met.
Turkish special forces entered the court house, which was evacuated, and gunshots were heard from inside the building, Turkish news agencies reported.
City police chief Selami Altinok told reporters that negotiations with the hostage takers were under way.
“We are trying to resolve the issue without anyone being hurt,” he said.
The Turkish government has banned live TV coverage of the incident, citing security concerns.