The jailed former Russian tycoon, Mikhail Khodorkovsky, has gone on trial at a court in Moscow on new charges of embezzlement and money laundering.
The former head of the disbanded oil firm, Yukos, is accused of embezzling 900bn roubles ($24.9bn) and could spend 15 more years in prison if convicted.
Once Russia's richest man, Khodorkovsky was jailed for eight years in 2005 for tax evasion and other offences.
His family has said the new charges are both politically motivated and absurd.
In a BBC interview, Khodorkovsky's mother Marinna dismissed the new accusations, saying they had only been brought to ensure her son remained in prison beyond the end of his first sentence.
"The authorities are afraid of him because he knows how corrupt they are," she said.
"He is also a very good leader, so those people are afraid that he will become the focal point of a new opposition movement."
Khodorkovsky was brought by guards to Moscow's Khamovnichesky district court on Tuesday morning for a preliminary hearing, the first time he has appeared in public in the capital since late 2005.
Wearing jeans and carrying a black briefcase, the former oil tycoon looked straight ahead and did not speak as he was led, handcuffed, into the building. He appeared in court inside a glass cage.
Several of his supporters were detained by police outside the courthouse after shouting "freedom for Khodorkovsky" and unveiling a banner from a nearby bridge.
Khodorkovsky and his former business partner, Platon Lebedev, were transferred to Moscow from their Siberian prison last week ahead of the hearing. Lebedev was also convicted of tax evasion at the same trial in 2005 and jailed for nine years.
Both men are accused by prosecutors of helping to embezzle some 900bn roubles of Yukos assets and launder 500bn roubles ($13.8bn) between 1998 and 2003.
One of Khodorkovsky's lawyers, Robert Amsterdam, told the BBC that his client denied all the charges and blamed senior officials close to Prime Minister Vladimir Putin, who was president during the first trial.
"The world is fairly universal in recognising the first trial as a show trial," he said.
"I believe these charges are, if it can be believed, even more fraudulent than the first set."
"One of the individuals who was to be put up as a witness was tortured and drugs were withheld from him," he added.
"He had Aids and his treatments were withheld in order to try to obtain perjured evidence, so I have absolutely no confidence that this trial would be any different than the first."
The Russian state prosecutors' office has refused to comment on the case.
The trial, which is closed to the public and media, is expected to last for at least several months.
Yukos, once Russia's biggest oil company, was declared bankrupt in 2006 and ceased to exist as a legal entity in November 2007.
The company had been steadily dismantled after being accused of massive fraud and tax evasion by the Russian authorities.
Yukos maintained it was the victim of a concerted political campaign by a government which wanted to discredit its executives and gain control of vital energy assets. Russian officials deny the allegation.