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.

Turkish police say a bomb explosion outside the office of a pro-Islamic magazine in Istanbul has killed one person and injured three others.

Police said Thursday that the device was placed outside the door of the monthly Adimlar magazine, on the third floor of a building, and exploded late Wednesday when the door was opened. Police identified the victim as 45-year-old Unsal Zor, who wrote for Adimlar, a magazine that is close to a Turkish militant Islamic organization.

A Turkish court has fined two leading cartoonists after convicting them of insulting President Recep Tayyip Erdogan in a caricature referring to the increasingly tough environment for journalists in the country.

Ozer Aydogan and Bahadir Baruter from the weekly satirical magazine Penguen were each initially sentenced to 11 months in prison by a criminal court in Istanbul on Tuesday.

Security footage appears to show three British girls, believed to be heading for Syria to join Islamic State (IS) militants, waiting at a bus station in Istanbul before traveling to a Turkish town on the Syrian border, media reported Sunday. 

Close friends Kadiza Sultana, 16, and 15-year-olds Shamima Begum and Amira Abase, boarded a flight from London to Istanbul on February 17. 

Turkish police on Friday arrested a man who threatened to set off a bomb outside the U.S. consulate in Istanbul, local media reported.

The suspect was not carrying any explosives, according to initial reports, the private Dogan news agency said.

Thousands of members of Turkey's Alevi religious minority on Sunday held a mass rally in central Istanbul, demanding greater rights and the preservation of the country's secular system.

The Alevis, who belong to an offshoot of Shia Islam, are Turkey's biggest religious minority, believed to make up at least 10 percent of the population.

Everything seems normal as the passengers stream out of a routine flight from a Middle Eastern country after landing at a major Turkish international airport.

But as they cross the air bridge from the plane into the airport, two plainclothes Turkish police -- on the lookout for jihadists on their way to Syria -- scrutinize their appearance and behavior.

Thousands of people marched though central Istanbul on Monday calling for justice over the murder of Turkish-Armenian journalist Hrant Dink who was shot dead in broad daylight eight years ago.

Holding signs in Turkish, Armenian and English reading "Justice for Hrant", they rallied around the offices of the Agos newspaper, a bilingual Turkish and Armenian weekly, which he edited.

Turkish police on Tuesday detained six people, including three foreigners, over last week's suicide bombing in the heart of Istanbul's tourist district, reports said. 

Six suspects were in court for  questioning by Turkish prosecutors over the deadly attack, the state-run Anatolia news agency reported.