* Riot police blamed by Ukraine for sniper deaths
* Russian security officials seen in Kiev
* Government under pressure to identify killers (Updates with prime minister's comments to Reuters)
By Pavel Polityuk and Alessandra Prentice
KIEV, April 3 (Reuters) - Ukraine's prime minister on Thursday blamed ousted President Viktor Yanukovich for the deaths of dozens of anti-government protesters shot by police snipers and urged Russia to hand him over to face charges.
Arseny Yatseniuk made his comments to Reuters after Ukraine's security service (SBU) blamed the killing of more than 100 protesters in mid-February on the Berkut riot police but said Yanukovich had been involved in planning the operation.
The SBU also said representatives of Russia's FSB security force had been at the SBU headquarters in Kiev - under the previous government - during three months of protests, and that Russia had flown explosives into Ukraine as they worsened.
The hints of Russian involvement could further strain ties with Ukraine's former Soviet master, which annexed the Crimea region after Yanukovich's removal from power in what has become the worst East-West standoff since the Cold War.
Yanukovich has taken refuge in Russia and denied ordering the shootings. Asked in an interview whether he held the deposed president responsible for the protesters' deaths, Yatseniuk said it was a matter for the prosecutor general to decide.
"But as a politician I can state that the former president is personally responsible and we would like to bring ... (him) to justice," he said. "It is unacceptable when the Russian Federation covers for a man who is under investigation for the charges of mass murder and crimes against humanity."
The new government has faced pressure to identify and punish the killers in an event which was a turning point in the Moscow-backed Yanukovich's ultimately doomed struggle to retain power.
"The former government of the country gave criminal orders and a huge number of people suffered in the 'mincer'," Interior Minister Arsen Avakov told a news conference that announced the initial findings of an investigation into the shooting.
Rooftop snipers picked off protesters and medical workers on Feb. 20. Many died on the spot from shots to the neck and scores of bodies were left strewn on the ground in central Kiev.
One day after the killings, which followed three months of mainly peaceful protests over Yanukovich's decision to spurn closer trade and political ties with the European Union, the president fled Kiev. He was deposed by parliament on Feb. 22.
The prosecutor general said 12 members of the Berkut had been detained on suspicion of shooting peaceful participants in the protests.
Acting Attorney General Oleh Makhnitsky said the detainees were members of a special force in the Berkut called the 'Black Unit' and the head of the SBU said members of a special unit within SBU had also been involved.
In late February the Interior Ministry disbanded the Berkut, whose name means golden eagle.
The presence of members of Russia's FSB, a successor of the KGB, at the SBU's headquarters in Kiev implied there had been active Russian involvement in events in Ukraine, SBU chief Valentyn Nalyvaichenko said.
"We have reasonable grounds to believe that these groups ... participated in the planning and implementation of the so-called anti-terrorist operation," he said.
The FSB denied any involvement. Russia's RIA news agency quoted it as saying: "Let these statements be on the conscience of the Ukrainian Security Services."
Nalyvaichenko also said that planes loaded with 5,100 kilograms of Russian-made explosives and other materials had landed at an aerodrome near Kiev from the Russian city of Chkalovsk in January.
"They brought the means of organising the shooting and destruction of our protesters on Maidan," he said, referring to the central Kiev square which became the heart of the protests.
Russia has denied any involvement in Kiev's unrest, suggesting that the protests were partly funded by the West.
Konstantin Dolgov, a Russian Foreign Ministry official responsible for human rights issues, said on Twitter it would be wrong to draw "hasty and politicised conclusions based on material and hypotheses that have 'suddenly' surfaced'".
The findings of Ukrainian security chiefs' investigation and the Berkut arrests go some way towards meeting demands for justice over the killings. But many Ukrainians, disillusioned with what they see as a corrupt and mismanaged political system, want more dramatic change.
"The authorities want to appease us with these arrests. They've not done enough and we see only cosmetic changes and we want radical reforms," said Ivan Kochmar, a 37-year-old resident of Kiev.
Anatoly Dekin, from Crimea, welcomed the moves, saying it would be "nonsense" if the killers were not convicted as they were "monsters who killed ordinary people".
Another Kiev resident, who gave his name only as Sergei, said the blame must be placed elsewhere: "The main people who gave the orders are from the Kremlin."
(Additional reporting by Paul Ingrassia and Richard Balmforth; Writing by Alessandra Prentice; Editing by Timothy Heritage and Giles Elgood)