LONDON (REUTERS) - Former Russian tycoon Mikhail Khodorkovsky warned Vladimir Putin on Thursday that the Kremlin chief was a "naked king" presiding over an economy that will weaken further, eventually provoking a battle between rival clans in Moscow.

"Putin with bare chest isn't a mighty leader: He is a naked king," Khodorkovsky, once Russia's richest man who was arrested in 2003 after falling foul of Putin only to be released in 2013, told an audience in London.

Khodorkovsky, who once controlled Russia's biggest oil company, said the Russian economy would weaken further and that eventually its weakness would provoke a battle between the powerful clans which dominate Russia.

In a message which is likely to anger the 62-year-old Kremlin chief, the former tycoon also called on the West to build ties with Russian opposition groups in expectation of an end to the Putin era.

"Tomorrow, when the regime changes, you will have to build a relationship and you will have very little time," said Khodorkovsky, 51.

"I know what I'm saying as I witnessed two revolutions in Russia."

Khodorkovsky, who once controlled the Yukos oil company and the Menatep group, was convicted of tax evasion and fraud in a Moscow trial which he said was motivated by enemies who wanted to rip apart his company.

He always denied the charges. The Yukos oil company was crippled with massive back-tax claims and then its main Siberian oil production units were sold off by the state, only to be bought later by state-controlled companies.

Khodorkovsky was pardoned by Putin in 2013 and left Russia.

He said Putin was in the twilight of his power but cautioned that it the finale of the Putin presidency which began on the last day of 1999 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," said Khodorkovsky.

A chemical engineer who served in the Communist Youth League, he started to trade goods as the Soviet Union crumbled but soon began buying up state assets, gaining control of some of Russia's best oil fields.

His wealth and power made Khodorkovsky one of Russia's most powerful oligarchs and his jailing made him Russia's most famous prisoner.

Putin has compared Khodorkovsky to US gangster Al Capone, a charge laughed off by Khodorkovsky's lawyers.