The Russian ambassador to Turkey was shot in the back and killed while giving a speech at an art gallery this evening.
Turkish security sources said the gunman, who shouted "Don't forget Aleppo", was an off-duty police officer.
The Russian foreign ministry confirmed the death of envoy Andrei Karlov, which marked one of the most serious spillovers of the Syria conflict.
Turkey has struggled with a string of attacks by Islamist and Kurdish militants but the killing of a Russian envoy could have resonance throughout the region.
Russia is a close ally of Syrian President Bashar al-Assad and its air strikes were instrumental in helping Syrian forces end rebel resistance last week in the northern city of Aleppo.
President Tayyip Erdogan, however, has been a vociferous opponent of Mr Assad.
Russia’s President Vladimir Putin has said that the killing of Mr Karlov was a provocation to try to spoil the Russia-Turkey ties and derail Moscow's attempts to find, with Iran and Turkey, a solution for the Syria crisis.
In televised comments, Mr Putin said he has ordered security at Russian embassies around the world to be stepped up and said he wanted to know who had "directed" the gunman's hand.
The attacker was smartly dressed in black suit and tie, and standing behind the ambassador as he made his speech at an exhibition, a person at the scene told Reuters.
"He took out his gun and shot the ambassador from behind. We saw him lying on the floor and then we ran out," said the witness, who asked not to be identified.
Two security sources identified the gunman as an off-duty police officer who worked in the Turkish capital.
A Reuters cameraman at the scene said gunfire rang out for some time after the attack.
The Anadolu news agency said the gunman had been "neutralised", apparently killed.
A video showed the attacker shouting: "Don't forget Aleppo, don't forget Syria!" As screams rang out, he could be seen pacing about and shouting as he held the gun in one hand and waved the other in the air.
Another photograph showed four people including what appeared to be the ambassador lying on the floor.
Russia and Turkey have both been involved in the conflict in Syria, which borders Turkey. Turkey has been a staunch opponent of Assad, while Russia has deployed troops and its air force in support of the Syrian leader.
Tensions have escalated in recent weeks as Russian-backed Syrian forces have fought for control of the eastern part of Aleppo, triggering a stream of refugees.
It was not immediately clear who was behind the attack.
Fighters from the so-called Islamic State militant group have been active in Turkey and carried out several bomb attacks on Turkish targets over the past year.
Minister for Foreign Affairs and Trade Charles Flanagan TD has condemned the killing. In a statement his evening, he said: "I condemn the brutal assassination of the Russian Ambassador to Turkey.
"I offer sincere condolences to Ambassador Karlov's family and loved ones and to the government and people of Russia. Diplomats play critical role in the conduct of relations between States and today is a bleak day for the diplomatic community around the world."
The US State Department, involved in diplomatic contacts with Russia in an attempt to resolve the refugee crisis unfolding around the city of Aleppo, has also condemned the attack.