A prominent Turkish-Armenian editor, convicted in 2005 of insulting Turkish identity, has been shot dead outside his newspaper's office in Istanbul.
Crowds of Hrant Dink's colleagues and supporters gathered at the scene, chanting their outrage at his murder.
Dink was given a six-month suspended sentence in October 2005 after writing about the Armenian "genocide" of 1915.
The US, EU and Armenia have condemned his murder and Turkey's leaders vowed to bring those responsible to justice.
"A bullet has been fired at democracy and freedom of expression," Turkish Prime Minister Recep Tayyip Erdogan told a hastily convened news conference.
The attack on Dink was an attack on Turkey and on Turkish unity and stability, Mr Erdogan said, adding that the "dark hands" behind the killing would be brought to justice.
The murder is likely to increase political tensions in Turkey, where politicians have been courting the nationalist vote ahead of presidential and parliamentary elections later this year, correspondents say.
Mr Erdogan said two people had been detained in connection with the killing but Turkish media later reported that they had been released.
Turkey's NTV television said police were searching for a teenager wearing a white hat and a denim jacket in connection with the murder.
The channel showed pictures of a white sheet covering the journalist's body in front of the newspaper building's entrance.
Dink, the editor-in-chief of the bilingual Turkish and Armenian weekly Agos newspaper, was one of Turkey's most prominent Armenian voices.
He was the frequent target of anger from Turkish nationalists who viewed him as a traitor, correspondents say.
Dink, 53, was found guilty more than a year ago of insulting Turkish identity after he wrote an article which addressed the mass killings of Ottoman Armenians nine decades ago.
Dink was gunned down in broad daylight
He always said his aim was to improve the difficult relationship between Turks and Armenians, but in one of his last newspaper columns, he admitted he had been getting deaths threats.
His computer hard drive was full of them, he wrote, amounting to what he called psychological torture.
Dink was among dozens of writers in Turkey who have been charged under 301 of Turkey's penal code with insulting Turkish identity, often for articles dealing with the killing of Kurds and Ottoman Armenians.
Hundreds of thousands of Armenians died in 1915 at the hands of Ottoman Turks. Armenians have campaigned for the killings to be recognised internationally as genocide. More than a dozen countries, various international bodies and many Western historians have done so.
Turkey admits that many Armenians were killed but it denies any genocide, saying the deaths were a part of World War I.
Turkey and neighbouring Armenia still have no official relations.