The Kremlin has rejected allegations that it was behind the murder of the journalist Arkady Babchenko.
Babchenko, a famed Russian war correspondent and outspoken critic of Vladimir Putin's government, was gunned down at his apartment in Kiev on Tuesday evening. He was 41.
Volodymyr Grysman, the Ukrainian prime minister, said immediately after the killing that he believed the "Russian totalitarian machine" was likely involved in the killing.
"I am convinced that the Russian totalitarian machine did not forgive him his honesty and principled stance," he said. "A true friend of Ukraine who was telling the world the truth about Russian aggression. His murderers should be punished."
Dmitry Peskov, a spokesman for Vladimir Putin, said on Wednesday that the suggestions of Kremlin involvement in the murder were "anti Russian smears."
"We strongly condemn this killing and hope for a real, and not a sham investigation into determining who ordered it," said Mr Peskov on Wednesday.
He added that Ukraine had become a "very dangerous place" for journalists, in an apparent reference to the unsolved killing of prominent liberal journalist Pavel Sheremet in 2016.
Arkady Babchenko shot to fame in the mid-2000s with a powerful memoir of his time in the Russian army during the Chechen wars of the 1990s.
He went on to become a prominent war correspondent and was known for writing bitterly critical Facebook posts about Mr Putin and his government.
Babchenko was shot several times at the doorway to his flat as he came home from a shop on Tuesday evening. He died in an ambulance on his way to hospital.
Ukrainian police say that they believe the killer had been lying in wait in the stairwell.
They have opened an investigation into premeditated murder, saying they believe the killing was likely motivated by Babchenko's "professional activities."
Russia's Investigative Committee has said it will launch its own investigation into the killing.
Memorial events were planned in Kiev and Moscow on Wednesday.
Appalled to see another vocal Russian journalist, Arkady Babchenko, murdered. My thoughts are with his wife and young daughter. We must defend freedom of speech and it is vital that those responsible are now held to account.
— Boris Johnson (@BorisJohnson) May 30, 2018
Babchenko, who fled Russia in 2017 saying he had received threats in the wake of an campaign of intimidation by pro-Kremlin media and politicians, is the latest of a number of outspoken critics and self-declared enemies of the Kremlin to be murdered in the Ukrainian capital.
Sheremet, a dual Russian and Belarusian citizen, died when his car exploded in the centre of Kiev in 2016 in still unexplained circumstances.
Denis Voronenkov, a former Russian MP who was gunned down in March last year after he fled to Ukraine and publicly condemned Russia's annexation of Crimea.
Amina Okueva, a Chechen who fought on the Ukrainian side against Russian forces in east Ukraine, was shot dead in October.
Her husband, Adam Osmaev, who Russian authorities have accused of plotting to kill Mr Putin, survived an earlier assassination attempt in the city.
The killings have prompted some Ukrainian officials to suggest that the Russian secret services are running an assassination program against regime critics and key Ukrainian military and intelligence officers in the city.
Ukraine and Russia are fighting a de-facto war in eastern Ukraine, where Moscow sent troops and equipment to support a separatist uprising in 2014. Fighting continues there despite a ceasefire signed three years ago.
Anton Gereshcenko, an aide to Ukraine's interior minister, wrote on Facebook after Babchenko's murder: "The Putin regime targets those it cannot break or intimidate."
He said the Ukrainian government should ensure the safety of Babchenko's family.
Sergei Lavrov, the Russian foreign minister, saying it was "very sad" Moscow had been accused of murdering the reporter.
"The Ukrainian prime minister is already talking about how it was done by Russian secret services," he said. "This fashion of conducting international affairs is very sad," Lavrov told reporters.
Boris Johnson said he was "appalled" by Babchenko's murder.
"Appalled to see another vocal Russian journalist, Arkady Babchenko, murdered. My thoughts are with his wife and young daughter. We must defend freedom of speech and it is vital that those responsible are now held to account," the Foreign Secretary wrote on Twitter.