Members of a banned group have taken a prosecutor hostage in his office in a court house in Istanbul, Turkish news agencies said.
Police said negotiators were speaking to two militants in an attempt to end the stand-off.
“There is no problem concerning any of our colleagues. 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.”
The private Dogan news agency said some shots were fired inside earlier in the incident. Police special forces entered the building and it was evacuated, it said.
The prosecutor has been named as Mehmet Selim Kiraz by the state-run Anadolu Agency and state television, TRT.
He is the prosecutor investigating the death of a teenager who was hit by a police gas canister fired during nationwide anti-government protests in 2013.
A website close to the left-wing DHKP-C group said militants from the banned organisation had taken the prosecutor hostage at midday.
It said they had given authorities three hours to meet five demands, including forcing policemen held responsible for the teenager’s killing to confess the death.
The group also demanded that the policemen be tried by “peoples’ courts” and that court officials drop prosecutions or investigations against people who took part in protests denouncing the boy’s death.
The website showed a picture of someone holding a gun to a man’s head with posters from the group in the background.
The DHKP-C, which seeks a socialist state, is considered as a terrorist group by Turkey, the US and the European Union.
The group has carried out sporadic attacks, including a suicide bombing on the US embassy in 2013 that killed a security guard. The group was more active in the 1970s.