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.

A Turkish court on Tuesday acquitted more than 200 army officers accused of plotting a coup against the Islamic-rooted government in 2003.

A total of 236 suspects in the so-called Sledgehammer (Balyoz) case were retried by an Istanbul court after Turkey's top court last year quashed their original convictions.

An armed leftist group took hostage Tuesday a Turkish prosecutor 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.

A massive power cut caused chaos Tuesday across Turkey, shutting down the metro networks in Istanbul and the capital Ankara, with the government saying an outside attack on the system was not ruled out.

The power cut, the worst in one-and-a-half decades, began around 10:36 am (0736 GMT) in Istanbul, the state-run Anatolia news agency quoted the Turkey Electricity Transmission Company (TEIAS) as saying.

"Now stand up, pray and repent," the man told Turkish photojournalist Bunyamin Aygun. "We are going to execute you tomorrow by the sword."

"I had all my life before me," he said. "I started dreaming how I would be executed every time I closed my eyes."

President Recep Tayyip Erdogan on Monday insisted he was still planning to visit Iran next week, despite a war-of-words with the Islamic republic triggered by the Yemen crisis and his accusations Tehran was seeking domination of the region.

Majority Sunni Muslim Turkey has said it supports the Saudi-led operation against Iran-allied Huthi Shiite rebels in Yemen to restore order in the country.

A Turkish court on Sunday charged two former policemen with illegally eavesdropping on President Recep Tayyip Erdogan following their extradition from Romania, state-media said. 

Sedat Zavar, a former police chief, and Ilker Usta, were detained at Istanbul airport late Saturday after their arrival on a flight from Bucharest, the Anatolia news agency reported. 

Turkey should adopt a presidential system, the country's Prime Minister has declared, refuting claims that he and President Recep Tayyip Erdogan were at loggerheads over the idea. 

The statement comes after a major rift between Erdogan and the ruling Justice and Development Party (AKP) in the run up to the June 7 parliamentary election where the AKP will seek to change the constitution to give Erdogan's office more executive powers. 

Ukraine government forces on Saturday accused pro-Russian separatists of using Grad multiple rocket launchers overnight in the country's war-torn east in violation of a truce deal signed last month.

Military spokesman Andriy Lysenko told reporters that separatists fired the heavy weapons towards the village of Novotoshkivka from their Lugansk stronghold.

A group of London teenage girls barred from travelling abroad attend the same school as three others who are thought to have gone to Syria, the High Court has heard.

Last week High Court judge Anthony Hayden barred five teenage girls from going overseas due to concerns they too would flee to Syria to join Islamist fighters.