The prosecutor for the case of Berkin Elvan, a teenager killed after being hit by a gas canister in the head during Gezi Park anti-government protests in 2013, has been taken hostage by a leftist group.
The Revolutionary People's Liberation Party/Front (DHKP/C) said Mehmet Selim Kiraz is held in his office at the Istanbul Courthouse and will be "punished by death" if its demands are not met.
The group wants justice for the teenager and that the policemen on trial for the killing issue a live confession. According to a statement carried on halkinsesi.tv, a site close to the group, the assailants have given authorities until 3.36pm to meet their demands.
Reuters reported that Turkey's special forces entered the courthouse building as people inside were being evacuated.
Berkin, then 14, was going out to buy bread when he was struck on the head by a teargas canister thrown by police during street battles with demonstrators in Istanbul. He died after 269 days in a coma.
Berkin's death, the eighth during the Gezi Park protests that shook the moderate-Islamist government of Recep Tayyip Erdogan, sparked a new round of protests across the country.
The extremist DHKP/C is a virulently anti-US and anti-Nato organisation that has waged a long-running war against the Turkish state and its security apparatus.
It claimed responsibility for the January suicide attack in Istanbul's tourist district of Sultanahmet that left the female bomber and a policeman dead.
In a statement on its website, the group said "our sacrificial fighter... carried out the sacrificial action on the tourist police department in Sultanahmet".
Sultanahmet is Turkey's most visited destination and comprises the historic sites of Aya Sofia and the Blue Mosque.
"It is the same state which shot Berkin Elvan and which protects the thief ministers," the statement continues, naming the suicide attacker as Elif Sultan Kalsen, a woman in her mid-twenties.
Established in 1978 as Devrimci Sol or Dev Sol, a splinter faction of the Turkish People's Liberation Party/Front, the group has targeted Turkish security and military officials as well as US military contractors.
The group launched rockets at the US consulate in Istanbul in 1992 and killed a prominent Turkish businessman in early 1996. The first suicide bombings took place in 2001. The most significant attack occurred in February 2013 at the US embassy in Ankara where a security guard was killed.