ISTANBUL/WASHINGTON - An off-duty policeman shot and killed Russia's ambassador to Turkey Monday night in Ankara in an apparent protest against Russia's deadly involvement in Syria.

The entire scene was captured on video.

Ambassador Andrei Karlov was making a speech at the opening of an art exhibition. The well-dressed gunman stood on the side of the stage, leading many in the audience to assume he was a bodyguard.

Without warning, he shot Karlov in the back, yelling "Allahu Akbar" and vowing to avenge what he said are Russian murders of innocent people in Syria. He pointed his pistol at the ceiling and briefly at the audience while the mortally wounded ambassador lay on the floor and tearful guests cowered behind tables.

A map showing the location of the exhibit hall in A map showing the location of the exhibit hall in Ankara where Russia's ambassador to Turkey, Andrei Karlov, was shot and killed Dec. 19, 2016. A map showing the location of the exhibit hall in Ankara where Russia's ambassador to Turkey, Andrei Karlov, was shot and killed Dec. 19, 2016.

One witness told VOA the gunman shouted "Do not forget Aleppo! Do not forget Syria! As long as our lands aren't safe, you will not be safe."

Three other people were wounded before security officers shot the gunman dead. Ambassador Karlov died at a hospital. He had been Russia's ambassador to Turkey since 2013.

Ankara Mayor Melih Gokcek identified the assassin as a 22-year-old police officer, Mevlut Mert Altintas.

Putin demanded to know "who directed the killer's hand," and is asking Turkey to step up security at Russian diplomat posts.

Turkish President Recep Tayyip Erdogan also called the killing a "provocation" and said "Turkey and Russia have the will not to be deceived."

Turkish Prime Minister Binali Yildirim praised the ambassador and said every effort is being made to bring those responsible for the attack to justice. The state-run Anadolu news agency said six people have been detained in connection with the investigation, including the roommate, parents and other relatives of Altintas.

Russian Ambassador to Turkey A man gestures after shooting Andrei Karlov, the Russian Ambassador to Turkey in this composite of three photos. A man gestures after shooting Andrei Karlov, the Russian Ambassador to Turkey in this composite of three photos.

White House spokesman Ned Price said the U.S. strongly condemns the assassination and sends its condolences to the victims.

"This heinous attack on a member of the diplomatic corps is unacceptable and we stand united with Russia and Turkey in our determination to confront terrorism in all of its forms."

Secretary of State John Kerry called it a "despicable attack," and U.N. Secretary-General Ban Ki-moon said he is "appalled by this senseless act of terror." Ban said there can be no justification for targeting diplomatic personnel and civilians.

President-elect Donald Trump called the murder of an ambassador "a violation of all rules of civilized order." He called the gunman a "radical Islamic terrorist," but Turkish officials have imposed a news blackout and have not provided any further details on the gunman.

The U.N. Security Council also condemned the attack.

Police create a barrier in Ankara after a gunman s Police create a barrier in Ankara after a gunman shot Russia's ambassador to Turkey at an art exhibition. (Y.Yazicioglu/VOA) Police create a barrier in Ankara after a gunman shot Russia's ambassador to Turkey at an art exhibition. (Y.Yazicioglu/VOA)

A Turkish official said there are "very strong signs" the gunman was a follower of Muslim cleric Fethullah Gulen, who lives in exile in the United States and is suspected of helping direct July's failed coup in Turkey.

A Gulen spokesman denied the cleric had anything to do with the assassination and told Reuters Gulen "categorically denies this heinous act."

Ties between Russia Turkey have been slowly improving since Turkish jets shot down a Russian bomber that strayed into its airspace from Syria last year.

Russian and Turkish officials have been working together to resolve the refugee crisis in Aleppo, caused, in part, by Russian airstrikes on Syrian rebels.

Also Monday, a man was arrested after firing shots near the entrance to the U.S. Embassy in Ankara. A State Department official said no one was hurt and thanked Turkish authorities for their "prompt response."

WATCH: Emergency vehicles respond to scene of shooting