Russian President Vladimir Putin has attended the memorial service for an envoy who was shot dead in the Turkish capital, Ankara by an off-duty policeman. Mourners gathered at the foreign ministry building in Moscow, where the memorial was being held amid heightened security.

Pallbearers carry the casket of slain Russian Ambassador to Turkey Andrei Karlov during the funeral ceremony at the Christ the Saviour Cathedral in Moscow, Russia, Thursday, Dec. 22, 2016. Karlov was shot dead by a Turkish policeman Monday in Ankara, Turkey.[Xinhua]

Russia has laid to rest its ambassador, Andrey Karlov who was gunned down in Ankara this week. High-level officials, including the Russian president, attended a memorial ceremony in Moscow to pay tributes to the killed ambassador.

"Today we bid farewell in the last journey of our colleague, friend, Andrei Gennadyevich Karlov, who while on duty became the victim of a despicable and vile terrorist act," Russian Foreign Minsiter Sergei Lavrov said.

Andrey Karlov's body was repatriated to Moscow on Tuesday. The same plane took 18 Russian investigators and Foreign Ministry officials to Ankara to participate in the investigation.

The Kremlin has so far refused to speculate whether Andrey Karlov's murderer acted alone or was part of a larger group. It said the murder was a blow to Turkey's prestige but said it wouldn't harm relations between the two countries.

Turkish President Tayyip Erdogan said there is 'no doubt' that the assassin was a member of the network of Muslim cleric Fethullah Gulen. The government refers to the followers of Gulen as the 'Gulenist Terrorist Network', or 'FETO'.

"There is no reason to hide that he's a member of the FETO network. All his connections, from where he was educated to his links, point to FETO," Erdogan said.

However, the exiled Turkish cleric Fethullah Gulen has denied any involvement in the killing of the Russian Ambassador. Vladimir Putin had posthumously given the ambassador the Hero of Russia award - the country's highest military medal.