Russian ambassador Andrei Karlov died in hospital after being shot in the back five times in a gun attack in Ankara
The gunman reportedly screamed in Turkish and Arabic before killing Mr Karlov, who had been delivering a speech for several minutes, when the shooter opened fire.
The killer shouted: "Allahu Akbar" before gunning the ambassador down.
Sources said the wounded ambassador remained inside the building alone with the gunman for more than 30 minutes.
The gunman, who is yet to be identified, carried on shooting Mr Karlov as special forces surrounded the building.
The Russian ambassador to Turkey has been shot in an attack in Ankara
The gunman, appeared to be a man in his 20s. He was smartly dressed in a black suit and was seen with his fingers and thumbs together listening intently to what the ambassador had to say.
He reportedly shot the ambassador five times in the back.
At least three other people have been shot and special forces were called to the scene as local media reported hostages were being held.
Security around the Russian embassy in Ankara has been stepped up after the Russian ambassador was shot by a gunman, according to Russian state media.
Local media broadcast images of the gunman
Images of the gunman have been posted on Twitter
The source said gunfire ended around 30 minutes later and the Turkish interior minister entered the building and found the wounded on the floor.
It was not immediately clear who carried out the attack. Islamic State militants have been active in Turkey and carried out several bomb attacks on Turkish targets.
A photograph posted on Twitter showed a man in a black suit holding a pistol, standing close to a podium in the gallery, its walls hung with pictures.
Hostages are reportedly being held in the building
Special forces have surrounded the building
Four people including a man who appears to be the ambassador lay on the floor.
Russian Foreign Ministry spokeswoman Maria Zakharova said Moscow was in touch with Turkish authorities about the attack.
She said: "Today, during a public event, an unknown person opened fire chaotically.
”As a result, Andrei Karlov, the ambassador to Turkey, received a gunshot wound."
People are seen lying on the floor in a photo posted online
Kremlin spokesman Dmitry Peskov told the Interfax news agency that President Vladimir Putin had been told about the attack and would be studying a report from the intelligence services and foreign ministry on the incident.
Mr Karlov was made Moscow’s ambassador to Turkey in 2013.
Russia and Turkey have been involved in conflict in Syria across the border from where over two million Syrian refugees have settled. Turkey has been a staunch opponent of President Bashar al-Assad while Russia has deployed troops and its air force in support of the Syrian leader.
The Russian ambassador was seriously wounded in the shooting
Ambulances and police cars lined up outside the art gallery
Speaking to Sky News about what sparked the attack, Chatham House Turkey expert Fadi Hakura said: “The relationship between Turkey and Russia over the situation in Aleppo.
“This increased co-operation between the two countries may have prompted this assassination attempt.”
He added: “I think that Russia will not take this lying down. I think Russia will intensify activities in Syria and Russian intelligence will track down the perpetrators of this assignation attempt.
“They will adopt a robust line.”
The ambassador, pictured moments before he was shot
He added that the attack will also lead to Turkey increasing its operations to track down Islamic State (ISIS) militants operating in the country.
The United States condemned the gun attack on the Russian ambassador to Turkey, the US State Department said.
US State Department spokesman John Kirby said: "We condemn this act of violence, whatever its source.
”Our thoughts and prayers are with him and his family."
Andrei Karlov was making a speech when he was shot in Ankara
Prime Minister Theresa May said the shooting is "a matter of concern" as she was told about the attack during a speech in the House of Commons.
Foreign Secretary Boris Johnson also left the chamber immediately after the Prime Minister's announcement.
During a statement about last week's European Council, Mrs May told MPs: "I was just looking at what I believe, Mr Speaker, is some breaking news that the Russian ambassador to Turkey has been shot, which is a matter which has yet to be confirmed, but I think is a matter of concern."