Updated 12.00pm: A senior Turkish prosecutor and his two hostage-takers were killed when security forces launched an operation to free the official in a bloody end to a six-hour standoff in Istanbul.

Two radical leftist militants had earlier taken the Istanbul prosecutor Mehmet Selim Kiraz hostage in his office, putting a gun to his head and threatening to kill him if their demands were not met.

Their demands related to the investigation Kiraz was leading into the killing of teenager Berkin Elvan, who died in March last year after spending 269 days in a coma from injuries inflicted by police in anti-government protests in the summer of 2013.

The authorities initially tried negotiating with the captors on a hugely tense day on Tueday in Turkey's largest city which also suffered a major power cut.

But they decided to launch an operation to free the prosecutor when gunfire was heard in their mobile phone communications with the captors, Istanbul police chief Selami Altinok told reporters outside the courthouse.

Altinok said the two "terrorists" had been killed. Kiraz was taken to hospital but rapidly succumbed to his wounds, doctors said in a statement on television.

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 had given an afternoon deadline for the prosecutor to identify the police officers who they say were behind the killing of Elvan and force them to make a "live confession" or he would be shot.