He was speaking as the capital marked the first anniversary of the clashes between protesters and police which toppled ex-President Viktor Yanukovych.
A top Kremlin aide, Vladislav Surkov, had organised snipers, he alleged.
The Russian government hit back at the claim, calling it ?nonsense?.
More than 100 people died in the violence on Kiev?s central Independence Square, known as the Maidan, a year ago.
The anti-Yanukovych revolt was called the ?EuroMaidan revolution?, as huge crowds demanded a pact with the EU.
In a speech at the Maidan on Friday, President Poroshenko condemned the insurgency by pro-Russian rebels in the east.
Earlier he told Maidan victims? relatives that, according to Ukrainian state security, ?the Russian presidential aide Vladislav Surkov led the organisation of groups of foreign snipers on the Maidan?.
He was speaking just two days after his army retreated from the key town of Debaltseve, now in rebel hands.
The Ukrainian government, Western leaders and Nato say there is clear evidence that Russia is helping the rebels with heavy weapons and soldiers. Independent experts echo that accusation. Moscow denies it, insisting that any Russians serving with the rebels are ?volunteers?.
A Ukrainian military spokesman, Andriy Lysenko, said more than 20 Russian tanks, 10 missile systems and busloads of troops had entered Ukraine in the past 24 hours, heading for Novoazovsk, a rebel-held town on the coast. The report has not been confirmed.
The Maidan ceremony includes poetry, a choir singing the national anthem, and a performance of Mozart?s Requiem by the National Symphony Orchestra of Ukraine.
Mr Yanukovych ? a political ally of Moscow ? fled into exile in late February 2014, but soon resurfaced in the southern Russian city of Rostov-on-Don.
He said he had been ousted in an ?illegal coup? and lambasted the ?fascists? who had taken power in Kiev.
For weeks in the bitterly cold winter of 2013-2014 the Maidan was a vast campsite populated by the EuroMaidan protesters, who kept police at bay with barricades and burning tyres.
Most of those killed in the Maidan clashes were shot by snipers, and some uniformed police were filmed firing at protesters.
A ceasefire for war-torn eastern Ukraine, signed in Minsk on 12 February, looks fragile as shelling continues in some areas. An intense rebel bombardment forced some 2,500 government troops to retreat from Debaltseve on Wednesday, and dozens of others surrendered.
The village of Chernukhino, near Debaltseve, is now in rebel hands too, the Kiev-appointed governor of Luhansk region, Gennadiy Moskal, said on Friday.
BBC
