A U.S.-based Muslim cleric has condemned the killing of Russia's envoy to Turkey and rejected accusations that his movement was behind the attack.

Ambassador Andrei Karlov was shot dead by an off-duty policeman in front of stunned onlookers at a photo exhibition in Ankara this week. Turkish President Recep Tayyip Erdogan has implicated Fethullah Gulen in the killing, saying the policeman had links to his movement.

In a video address made available to the Associated Press, Gulen accused the Turkish government of blaming and defaming his movement and suggested the government would facilitate other assassinations and blame them on his followers.

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Islamic cleric Fethullah Gulen, seen in July of 2016, said in a video released to The Associated Press that the Turkish government was trying to defame his movement with the assassination allegation. (Chris Post/The Associated Press)

Gulen, who lives in Pennsylvania, said "it is not possible for them to convince the world of such accusations."

Gulen has already been blamed by Erdogan for inciting the failed military uprising in July that led to the imposition of a state of emergency, an extension in pre-trial detention, the arrest of around 38,000 people and the purging of more than 100,000 from government jobs.

Whatever motivated gunman Mevlut Mert Altintas, an off-duty policeman killed by police after shooting Karlov at a photo exhibition in Ankara on Monday, has not been confirmed. He shouted religious phrases and appeared to condemn Russian bombardments of rebel-held neighbourhoods of Syria's Aleppo city — the shooting followed protests by Turks who criticized Russia's support for Syrian President Bashar al-Assad.

Turkey's state-run news agency confirmed early Thursday that authorities released the family members of Altintas.

The policeman's parents, sister and three other relatives were among 11 people detained in the wake of the fatal shooting.

The Anadolu Agency says the six family members were released without charges. Those still in custody include Altintas's roommate.

Putin, Lavrov attend ceremony

Meanwhile, in Moscow a sombre memorial ceremony was held for Karlov.

Diplomats and family members gathered at the Russian Foreign Ministry, a looming Stalin-era skyscraper in central Moscow, to bid farewell to Karlov, who was 62.

TV footage showed Karlov's body lying in an open casket in the building's marble-clad lobby flanked by a uniformed honour guard as mourners, including Prime Minister Dmitry Medvedev and Foreign Minister Sergei Lavrov, approached to lay flowers.

Lavrov said Karlov had been murdered on the front line and had been the victim of "a despicable terrorist act."

President Vladimir Putin also paid his respects, briefly sitting beside the coffin and speaking to Karlov's widow, who wore a black veil. Karlov was a Soviet-trained diplomat who worked in North and South Korea during the 1990s and 2000s and was sent to Turkey in 2013.

His name was etched into a slab of pink marble on the wall of the Foreign Ministry along with the names of Russia's most illustrious diplomats.

Proceedings will move to Moscow's gold-domed Christ the Saviour Cathedral later on Thursday where the head of the Russian Orthodox Church will lead a ceremony.

Putin, who has said he knew Karlov personally, has posthumously given Karlov the Hero of Russia award, the country's highest military medal.

"We must know who directed the killer's hand," Putin said after the murder, promising retribution.