Two armed people entered an office of Turkey's ruling AK Party in an Istanbul suburb on Wednesday, the private Dogan news agency reported, a day after leftist militants took a prosecutor hostage in a courthouse in the city.
Dogan said police sealed off the road in front of the building in the Kartal district on Istanbul's Asian side. Footage on Dogan's website showed several police cars outside the building. No further details were immediately available.
Turkish hostage crisis: Prosecutor dead
The Istanbul prosecutor held hostage by two radical leftists at his offices has died of his wounds sustained after police launched an operation to free him.
Prosecutor Mehmet Selim Kiraz "was severely injured when he arrived (at the hospital) but we lost him despite all our efforts," hospital doctors said in statement broadcast on national television.
An armed leftist group took hostage the Turkish prosecutor, who was probing the politically sensitive death of an anti-government protester, threatening to kill him if their demands were not met.
Gunfire was heard at the Istanbul courthouse where the prosecutor, Mehmet Selim Kiraz, was working, the Dogan news agency reported said.
Kiraz was investigating the killing of Berkin Elvan, who died in March last year after spending 269 days in a coma due to injuries inflicted by police in the mass protests of early summer 2013.
The Istanbul police chief had told reporters outside the courthouse that talks were in progress with the hostage-takers and said he hoped the standoff would end peacefully.
"God willing this protest will end without having any of our friends hurt," Istanbul police chief Selami Altinok said.
In a statement published by Turkish media, the hostage-taking was claimed by the radical Marxist Revolutionary People's Liberation Party?Front (DHKP-C), which has been behind a string of attacks over the last years.
The group published pictures showing one of the militants -- his face concealed by a scarf with the group's red and yellow insignia -- holding a gun to Kiraz's head.
They had also plastered their flags and posters on the walls of his office which is on the fifth floor of the courthouse building.
Turkish media said that the group had given a deadline of 3:36 pm (1236 GMT) for the prosecutor to identify the police officers who they say were behind the killing of Elvan or he would be shot.
Their demands also included a "live confession" by the officers responsible and that they should be put before a "people's court".
Kiraz, wearing a suit and tie, is made to stare into the camera as the militant puts his hand round his chin. His mouth appears to have been bound with duct tape.
Special forces were waiting outside the court house building where the prosecutor was taken hostage amid expectations an operation could be launched.
Police moved onlookers away from that building, warning the hostage takers could start firing from the windows.
The Turkish television watchdog ordered a media blackout on reporting of the standoff, forcing Turkish television channels to cut the live transmissions of the incident.
'Blood can't be washed'
Elvan succumbed to injuries sustained when he was hit by a tear gas canister fired by police in the mass protests of early summer 2013 against the government of President Recep Tayyip Erdogan who was then premier.
He was aged 15 at the time of his death, which was followed by mass nationwide protests that were also largely put down by the police.
Elvan has since become an icon for the Turkish far-left and his supporters accuse the authorities of covering up the circumstances and perpetrators of his death.
Despite the investigation, no police officer has been brought to trial over the his death, to the fury of Elvan's supporters.
In a phone call with opposition lawmaker Huseyin Aygun, Berkin Elvan's father Sami urged against bloodshed, the Hurriyet newspaper reported on its website.
Elvan said: "My son died but I don't want any other person to die. The prosecutor must be released. Blood cannot be washed away with blood."
The DHKP-C is considered a terrorist group by Turkey, the European Union and the United States.
Earlier this year, it claimed a January 1 attempted grenade attack on police guarding the the Dolmabahce palace in Istanbul that caused no serious casualties, saying it was revenge for Elvan's killing.
It also claimed a suicide attack in Istanbul later that month but amid a bizarre sequence of events then withdrew the claim, saying it had made a mistake.
The hostage-taking coincided Tuesday with the worst power cut Turkey has seen for 15 years, which has caused chaos in Istanbul.
There was no indication of a link between the two events although some reports said the armed radicals had been able to enter the courthouse as the metal detectors were not working due to the electricity stoppage.
Erdogan had sparked an outcry after he called Elvan a thug with links to a "terrorist organisation" and encouraged his supporters at a rally to boo Elvan's mother.