Denis Voronenkov was killed as he walked along the road in Kiev, Ukraine, police have said.
Shocking pictures show the dead man bleeding out as members of the public look on.
He and his security guard were reportedly murdered by an unidentified gunman at the entrance of an upmarket hotel in the Ukrainian capital.
oronenkov, 45, was an ex-member of the communist faction in the lower house of the Russian parliament.
He fled to Ukraine last Autumn after fearing for his life as an outspoken critic of Russian President Vladimir Putin.
He famously once compared Putin’s government to “Nazi Germany”.
The former MP left Russia with his wife, singer Maria Maksakova, who was also involved in politics.
Voronenkov was reportedly on his way to meet Ilya Ponomarev, also an exiled ex-Russian MP.
Ponomarev wrote on Facebook: “I have no words. The security guard was able to injure the attacker.
“The potential theory is obvious. Voronenko was not a crook, but an investigator who was fatally dangerous to Russian authorities.”
Kiev’s police chief, Andriy Grishchenko, said he could “confirm” the assassination.
He added that the injured killer has been caught but gave no further details of his identity, saying: “The killer is under protection, he is given priority medical assistance.”
However, just hours later, police sources confirmed the suspected assassin had died in hospital.
Voronenkov renounced his Russian citizenship after he fled in October 2016, saying he was forced to leave after being persecuted by Russian security agencies.
The killing will raise fears of tensions between Russia and Ukraine deepening.
The countries have been at loggerheads since the annexation of Crimea in 2014 by Russian forces.
Ukraine’s president, Petro Poroshenko, called the head of the security services for a meeting just after the killing.
And Ukraine’s prosecutor general, Yuri Lutsenko, wrote on social media that the murder was “the usual kind of Kremlin retribution”.
But the Kremlin has released a statement denying any involvement, saying accusations of the like are “absurd”.
Police sources in Kiev also claimed the Russian secret services may be behind the shooting, adding was a key witness in a treason case against deposed Ukrainian president and Putin ally Viktor Yanukovych.
Anton Herashchenko, an adviser to the Ukrainian Interior Ministry, said: “It is an ordered political murder, made for the purpose of intimidating all those Russian deputies and officials who try to escape from Russia to the West or to Ukraine.”
The aim was to gag people like him who try to “tell the truth about how the punitive Russian apparatus works”.
He added: “He was killed in retaliation for the fact that he and his wife gave truthful testimony about how the annexation of the Crimea and the attack on the Donbass was prepared.”
Ms Maksakova fainted after arriving at the scene of the shooting.
She is believed to be pregnant.
President Putin is accused of being behind the deaths of numerous vocal critics.
Former Russian secret service officer Alexander Litvinenko is one of the most high-profile assassinations linked to the Russian leader.
In 2006, the London-resident was poisoned using the radioactive substance, polonium-210 – supposedly slipped into a drink by Russian spies.