Several media outlets reported that a gunfight ensued after Karlov was shot.
Local broadcaster NTV television said at least three people were wounded and were taken to the hospital.
Mayor Gokcek told reporters outside the exhibition centre that the “heinous” attack was aimed at disrupting newly re-established relations between Turkey and Russia.
Relations between Russia and Turkey were badly strained by the downing of a Russian warplane at the Syrian border in November 2015, but Turkey’s apology earlier this year helped overcome the rift.
The leaders of the two countries, Presidents Vladimir Putin and Recep Tayyip Erdogan, have held several meetings in recent months and spoken frequently over the phone. The two leaders have already spoken by phone about Monday’s attack.
Russia and Turkey have co-sponsored the evacuation of civilians and rebels from Aleppoand discussed the prospect of organising a new round of peace talks in Kazakhstan’s capital, Astana.
Russia’s ambassador to Ankara has been killed in a gun attack in the Turkish capital, Ankara, Russia’s foreign ministry said.
Police later killed the assailant, Turkish station NTV reported.
Foreign Ministry spokeswoman Maria Zakharova made the announcement of Karlov’s death in a live televised statement.
The assailant was a 22-year-old off-duty police officer who worked in the Turkish capital, said Ankara’s mayor Melih Gokcek.
After the initial shot, the attacker approached Karlov as he lay on the ground and shot him at least one more time at close range, according to an AP photographer at the scene.
He also smashed several of the framed photos hung for the exhibition but later let the stunned guests out of the venue, according to local media.