A former Russian lawmaker and key witness in a treason case against Ukraine's ousted leader Viktor Yanukovich was shot dead in broad daylight outside a hotel in central Kiev on Thursday, police said.

Former MP Denis Voronenkov was killed by an assailant who is now in hospital after being wounded by Voronenkov's bodyguard during the incident, police said.

Voronenkov fled to Ukraine with his wife last year and was helping the Ukrainian authorities build a treason case against former Ukrainian president Yanukovich.

He has also spoken out against Russia's annexation of Crimea in March 2014, although he voted for the move at the time.

"A cynical murder of one of the witnesses involved in the state's case against ex-president Yanukovich," said Ukraine's General Prosecutor Yuriy Lutsenko.

"In broad daylight in the centre of Kiev, former Russian lawmaker Denis Voronenkov was shot. He had provided investigators of the military prosecutor's office with highly important (witness) testimony for the case. This was a typical show execution of a witness by the Kremlin." 

Voronenkov had been placed on a Russian federal wanted list in connection with an alleged $5 million property fraud.

Yanukovich fled Ukraine during the 2013-2014 Maidan street protests, which he said were tantamount to a "coup" organised by armed nationalist radicals.