Chelsea soccer club owner Roman Abramovich hit back at rival Russian oligarch Boris Berezovsky in a $6 billion London court battle, accusing him of megalomania and pursuing a political career to promote his own interests.

The two men made their fortunes in Russia in the mid-1990s when a small group of businessmen snapped up shares in companies sold off after the collapse of the Soviet Union and fell out a decade ago.

However, the case which started in early October in London's Commercial Court, is followed closely by Russia watchers for clues about the contemporary state of play in the intertwined worlds of business and politics in Russia.

It was a desire for power that drove Berezovsky's political activities in Russia, Abramovich said in a witness statement made public Monday, when he began giving evidence in a courtroom swarming with bodyguards wearing earpieces and sunglasses and armies of lawyers and aides.

There was at times something of a megalomaniac about him that could lead to extraordinary or even fantastic suggestions on his part, Abramovich wrote in his statement, adding that one of Berezovsky's ideas was to restore the monarchy in Russia.

Once a close ally of former Russian president Boris Yeltsin, 65-year-old Berezovsky fled to Britain in 2001 after falling out with Yeltsin's successor Vladimir Putin.

Criminal charges were also brought against Berezovsky in Russia over his business dealings.

He suggests that his falling out with President Putin and his resignation from the Duma (lower house of Russian parliament) were as a result of his supposed commitment to a democratic Russia, Abramovich wrote.

The fact that Mr Berezovsky always had a lot of ideas about the democratic future of Russia is beyond dispute. Nonetheless, his political career never seemed to me to be a model of selflessness.

Berezovsky, a vocal critic of the Kremlin who has even threatened to topple Putin, says Abramovich intimidated him in 2000 into selling shares in Russian oil company Sibneft at a fraction of their value. He made no remarks in court Monday.

Abramovich, 45, whose wealth has helped to transform the fortunes of London soccer club Chelsea, denies that Berezovsky ever had an interest in Sibneft.

It was in return for Berezovsky's political patronage that the then up-and-coming businessman Abramovich paid him at least $2 billion between 1995 and 2002, Abramovich's lawyer Jonathan Sumption said earlier this month.

The cross-examination of Abramovich, who rarely speaks in public, followed hearings of Berezovsky's evidence.

Abramovich, who spoke in Russian via an interpreter, looked calm and emotionless -- in contrast to the bubbly Berezovsky -- and often asked Berezovsky's lawyer to repeat or rephrase his questions.

Berezovsky says his decision to sell his Sibneft stake was influenced by fears that, if he refused, Abramovich would ensure Putin intervened and the shares would be expropriated.

Abramovich sold a controlling stake in Sibneft to Russian gas giant Gazprom for more than $13 billion in 2005.

The next court hearing is scheduled for Tuesday.

(Reporting by Olesya Dmitracova)