Mikhail Kasyanov has said he fears for his life after fellow activist Boris Nemtsov was gunned down in Moscow last year, leaving him as the most high-profile Kremlin opponent inside the country.
Kasyanov said he had received multiple death threats in recent months, but would not flee his native Russia, where he wants to contest elections and hopes for a change in power in as little as two years.
"I am afraid for my life and for those of my colleagues," Kasyanov told Reuters in an interview at the Moscow headquarters of his People's Freedom or PARNAS party. "Anyone normal would fear for their life."
Kasyanov was shown in the cross-hairs of a sniper's rifle in a video posted on the internet earlier this month by the pro-Russian boss of Chechnya, a Putin ally known for scathing condemnations of the Kremlin's enemies.
Days later, two men accosted Kasyanov in a Moscow restaurant, rubbed a cake in his face and threatened him.
Kasyanov, who is co-chairman of PARNAS, said the Kremlin and nationalist groups had designated him as "enemy number one" now that Nemtsov was dead and because of his decision to head his party's list in parliamentary elections in September.
"The authorities want to stop my political activity with threats," he said. "They want to force me out of the country. Their aim is to frighten me. But I have decided that I am going to continue with my activity."
Kasyanov was hounded by pro-Kremlin activists in recent weeks when he tried to address voters in Russia's regions and says his assistants were physically assaulted. Footage of one incident shows him being chased down a corridor.
He said he was the only former prime minister who did not have bodyguards assigned to him by the state and that Putin had personally stripped him of his security detail.
The Kremlin did not immediately respond to an email seeking a response to Kasyanov's assertion that he was in danger.
In the past, the Kremlin has described threats against Kasyanov as ordinary hooliganism or said it was unaware of the specifics. Police have said they are looking into recent incidents, but have so far not said whether they will open a criminal case.
Once on good terms with the Russian leader until Putin fired him and his cabinet in 2004, Kasyanov said he thought Putin had started off pretending to be a democrat before showing his true colours.
What followed, he said, was the systematic suppression of freedom of the press, the judiciary, and the introduction of Putin's "vertical" system of rule, in which all officials ultimately report to the Kremlin.
Now, Kasyanov said, he thought Putin had two choices: either to step down when his term expired in 2018 or to go for what would be a fourth presidential term and struggle to keep a lid on the unrest that Kasyanov predicted would erupt.
"It all depends on Putin," said Kasyanov. "Either he stops his aggressive policy in and outside the country and starts to relax things and draw up an exit strategy for himself or he ... will end up provoking clashes that could end very badly."
Kasyanov said Russians could reach a "revolutionary state" exacerbated by an economic crisis in as little as two years.