Former Russian tycoon Mikhail Khodorkovsky has likened Vladimir Putin to a naked king presiding over a weak economy that would weaken further and provoke wars between Russia's powerful clans.
"Putin with bare chest isn't a mighty leader: He is a naked king," said Khodorkovsky, who once controlled Russia's biggest oil company Yukos.
Khodorkovsky, once Russia's richest man was arrested in 2003 for tax evasion and fraud after falling foul of Putin, who had back then compared him to American gangster Al Capone.
As one of Russia's most powerful oligarchs, Khodorkovsky's jailing made him Russia's most famous prisoner.
When pardoned by Putin and released in 2013, he left Russia.
He has denied the charges levelled against him.
Addressing an audience in London, the tycoon also called on the West to build ties with Russian opposition groups in expectation of an end to the Putin era, reports Reuters.
"Tomorrow when the regime changes you will have to build relationship and you will have very little time," said Khodorkovsky, 51. "I know what I'm saying as I witnessed two revolutions in Russia."
He said Putin was in the twilight of his power and that the end of the Putin presidency could be painful and dangerous for both the West and Russia.
"Russia will have to stand long in agony by the bed of the sick emperor. It will be painful for the population and dangerous for the West," Khodorkovsky said.
US President Barack Obama recently said the Russian economy was in tatters, following economic sanctions and falling oil prices.
The rouble has lost nearly half of its value in the past 12 months and the government expects the economy to shrink 3% this year. Moody's has lowered ratings of six Russian financial institutions which are branches of foreign banks.
But despite all that and predictions of a 10% drop in gross domestic product this year notwithstanding, Putin saw his ratings at home go up 86% in a recent poll survey done by the Levada Center.