Bill Browder, chief executive and founder of Hermitage Capital Markets, told CNBC the Russian government is "apoplectic" over sanctions imposed on Russian officials that he has campaigned for, and if he gets assassinated, "everyone would know who did it."
The well-known critic of the Kremlin has been living in London since he was kicked out of Russia for accusing Russian tax officials of embezzlement, in 2007.
Since then, he has repeatedly accused Russia of corruption and has been involved in a high-profile battle with the Russian state over the death of his lawyer Sergei Magnitsky, who was investigating fraud among Russian officials.
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In April, Russia issued an arrest warrant against Browder on charges that he stole shares in gas giant Gazprom fifteen years ago and requested Interpol, the global police agency, to launch a manhunt for the investment banker.
Interpol refused the request but Browder said he feels under constant threat.
"At the moment nothing keeps me protected…If I get assassinated, everyone will know who did it….It would effectively be a declaration of war with the West if they decide to kill me," he told CNBC in London.
Browder's colleague Magnitsky was imprisoned and taken ill, and he died later in hospital. Browder has since sought to bring the Russian state to trial over Magnitsky's death.
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His campaign has not fallen on deaf ears with the U.S. signing the Magnitsky Act into law in December 2012. The Act prohibits Russian officials thought to be connected to the death of Magnitsky from entering the U.S and using its banking system, a factor that has enraged Russia, Browder said.
"In December 2012, the Magnitsky Act was passed which imposes sanctions on Russian officials and in April 2013, they actually sanctioned 16 officials [on the U.S. OFAC sanctions list] and the Russians are apoplectic because this is such a blow to their reputation."
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He said the Russian president had become "the biggest oligarch" since coming to power but that social unrest was intensifying. This week, thousands of protestors marched through Moscow calling for what they see as Putin's authoritarian rule and for the release of people considered political prisoners.
- By CNBC's Holly Ellyatt, follow her on Twitter @HollyEllyatt