Mikhail Khodorkovsky has resigned as head of Russian oil giant, Yukos. The tycoon, who is reportedly the country's richest man, was arrested at gunpoint a week ago on charges of fraud and tax evasion. The government also froze a controlling stake of the company's shares. Yukos has been at the centre of a confrontation between big business and the Kremlin. The episode is raising concerns that President Vladimir Putin is becoming increasingly authoritarian in the run-up to parliamentary elections in December and a Presidential vote in March. It is also highlighting schisms within the government, Chief of Staff, Alexander Voloshin, resigned over Khodorkovsky's arrest. Prime Minister Mikhail Kasyanov voiced his concern over the freezing of Yukos shares. Critics have interpreted Khodorkovsky's arrest as an attempt by the Kremlin to quash his political drive. The oil tycoon has openly backed the opposition and is believed to have presidential ambitions. Following Khodorkovsky's resignation Yukos shares rose by 4.1 percent - reflecting investor hopes the affair draws quickly to a close. **PHOTO CAPTION*** Russia's richest man, Mikhail Khodorkovsky, has quit Yukos. (AFP/File/Denis Sinyakov)