Ukraine has accused Russia of "state terrorism" after a former Russian politician and key witness in a treason case against former leader Viktor Yanukovich was shot dead in broad daylight outside a hotel in central Kiev on Thursday.
Russia called the allegation "absurd".
Former MP Denis Voronenkov was killed by an assailant who was armed with a pistol and is now critically ill in hospital after being wounded by Mr Voronenkov's bodyguard, police said.
Mr Voronenkov fled to Ukraine last year and was helping the Ukrainian authorities build a treason case against Mr Yanukovich, Ukraine's pro-Russia former president.
Mr Voronenkov had also spoken out against Russia's annexation of Crimea in March 2014, although he voted for the move at the time.
President Petro Poroshenko said the killing "is an act of state terrorism on the part of Russia, which (Mr Voronenkov) was forced to leave for political reasons."
"Mr Voronenkovwas one of the main witnesses of Russian aggression against Ukraine and, in particular, the role of Mr Yanukovich regarding the deployment of Russian troops to Ukraine."
Relations between Kiev and Moscow are at an all-time low after Russia's annexation of the Crimean peninsula in March 2014 and the subsequent outbreak of separatist fighting in Ukraine's eastern Donbass region that has killed more than 10,000 people.
Mr Poroshenko said it was "no accident" that Mr Voronenkovwas shot on the same day as a warehouse storing tank ammunition was blown up at a Ukrainian military base.
Moscow denied any involvement Mr Voronenkov's murder .
"We believe that all the falsehoods that can already be heard about much-hyped Russian involvement are absurd," Kremlin spokesman Dmitry Peskov was quoted as saying about the killing.
Mr Voronenkov, 45, had been placed on a Russian federal wanted-list in connection with an alleged $5 million property fraud. He came to Ukraine with his wife, opera singer Maria Maksakova, who was also an MP.
Mr Voronenkovwas gunned down on his way to meet another former Russian parliamentarian, Ilya Ponomarev, who was the only member of the Duma who voted against the annexation of Crimea.