Canadian Prime Minister Justin Trudeau ended a surprise trip to Ukraine with a fierce attack on Vladimir Putin, describing the Russian leader as "weak" who uses the police and army to crush his opponents.

Speaking to reporters at the end of his visit to the capital Kyiv, Trudeau accused Putin of "executing" opposition leader Alexei Navalny, who unexpectedly died a week ago in a penal colony in the Arctic where he was serving a 19-year prison sentence.

Navalny, 47, was widely regarded as Putin's biggest political enemy, and the Kremlin rejected allegations that Putin was involved in Navalny's death, describing them as "blatant, baseless accusations against the Russian head of state."

Trudeau also said on Saturday: "I think we know, and have seen repeatedly, how any opposition in Russia is marginalized or openly executed."

He continued: "What happened to Alexei Navalny shows that despite all the bravado Putin displays, he is in reality a coward."

He added: The execution of political opponents, the suppression of opposition using police and the army, and making sure there is no opposition, is a sign of weakness, not a sign of a confident person in his position.

Putin will run for a fifth term as president next month, and Trudeau launched his attack on the Russian leader when asked whether he would recognize the results of those elections.