The jailed Russian oil tycoon Mikhail Khodorkovsky has gone on hunger strike in protest at an order extending his detention ahead of a new trial.
In a statement quoted by his lawyer, Khodorkovsky said the order violated legal amendments that had been approved by President Dmitry Medvedev.
He has vowed to continue his protest until President Medvedev confirms that he is "fully aware" of the situation.
Khodorkovsky is serving an eight-year jail term for tax evasion and fraud.
His lawyer Vadim Klyuvgant said a law signed by Mr Medvedev in March banned the detention of suspects charged with economic crimes.
On 14 May a Moscow court extended Khodorkovsky's detention by three months, Interfax news agency reports.
Khodorkovsky's complaint was submitted to the head of Russia's Supreme Court. It was also written on behalf of his former business partner Platon Lebedev, whose detention was also extended.
Khodorkovsky, former head of energy giant Yukos, was once Russia's richest man.
He has been in prison since 2003 and has previously staged several brief hunger strikes.
Now he faces new theft and embezzlement charges that could land him with 22 more years in jail.
Yukos first faced allegations of tax fraud in 2002 and was ultimately liquidated in 2007.
But the firm claims that it was "targeted" by the Russian authorities and illegally driven out of business.
The legal changes approved by President Medvedev clearly state that people charged with economic crimes can only be held in prison under very specific circumstances - for example, if their identity is not known or if they have a history of trying to flee, the BBC's Richard Galpin reports.
Although Khodorkovsky has not finished his current prison sentence he has been eligible for parole for the past three years.
He has always argued that he is the victim of a political witch-hunt.