ISTANBUL  (AFP) -- Turkish authorities on Wednesday rounded up over 30 suspected members of the radical leftist group behind a bloody hostage standoff that left a top Istanbul prosecutor dead and shocked the country.

Funeral ceremonies were being held for prosecutor Mehmet Selim Kiraz, who had been investigating the politically-sensitive case of a teenager who died of injuries inflicted by police during anti-government protests in 2013.

Police late on Tuesday launched an operation to free Kiraz after an hours-long standoff with his captors but the official, who had sustained multiple gunshot wounds to the head and chest, died shortly after arriving at hospital.

It was not clear from where the shots that killed him were fired.

Both his captors, two men in their 20s affiliated to the outlawed Marxist Revolutionary People's Liberation Party-Front (DHKP-C), were killed in the police operation.

Turkish authorities on Wednesday detained 22 suspected members of the group in the southern city of Antalya after receiving a tip-off they were planning further attacks, the Dogan news agency reported.

Police in the western city of Izmir also detained 5 suspected DHKP-C members, seizing documents, digital recordings, banned magazines and 30 bullets. Five more people were also detained in Eskisehir, reports said.

The DHKP-C is considered a terrorist group by Turkey, the European Union and the United States and has carried out a string of attacks in Turkey in the past.

Turkish police also arrested an armed man who stormed an office of the ruling party in an Istanbul district but the incident was not believed to be related.

A ceremony to remember Kiraz was held at the Istanbul Caglayan Palace of Justice where he worked and the hostage drama unfolded.

Hundreds of lawyers, prosecutors and staff stood in respect on every floor of the giant building -- said to be the largest courthouse in Europe -- and unfurled a giant Turkish flag from the top floor.

"We will not forget you, our martyr," read a gigantic banner.

Prime Minister Ahmet Davutoglu was to be among the mourners at the funeral at the Eyup Sultan Mosque in Istanbul on the banks of the Golden Horn, with police snipers placed on the balconies of minarets amid tight security.

President Recep Tayyip Erdogan, currently on a visit to Romania, would return home early and miss the official dinner in Bucharest, the official Anatolia news agency said. 

Turkey's press expressed anguish over a "Black Tuesday" which also saw the worst nationwide power cut in the country in 15 years.

"The day we went into darkness," said the daily Cumhuriyet.

Kiraz had been leading a hotly-politicised investigation 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.

So far no-one has been brought to trial for the crime and the captors demanded that Kiraz hand over the names of the suspects and force them to confess.

Publishing photos of the prosecutor with his mouth bound and a gun to his head, they threatened to kill him if their demands were not met.

Davutoglu said it was an "attack against the Turkish judiciary, Turkish democracy and all the citizens in Turkey."

Turkish media reports said the attackers had carried fake lawyers' ID and had legal robes, allowing them to get round security checks.

Kiraz pressed a panic button after he was seized but the attackers had already locked the doors, the Milliyet daily said.

The circumstances of the police operation were not immediately clear but Istanbul police said it was launched after they heard gunshots coming from the office where he was held.

Energy Minister Taner Yildiz vehemently denied there was any link between the hostage drama and the power cut, after opposition leader Kemal Kilicdaroglu claimed the power could have been cut intentionally to assist the captors.

The drama came at a time of intensifying political tensions in Turkey ahead of June 7 legislative elections.

Erdogan's ruling Justice and Development Party is seeking a landslide victory, which would allow it to change the constitution to boost the powers of the presidency which he assumed in 2014 after over a decade as prime minister.