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MOSCOW — Mikhail B. Khodorkovsky appeared in a Moscow courtroom on Tuesday to face new charges in a politically fraught trial that could prolong his time in prison by more than two decades.

Mr. Khodorkovsky, the former billionaire owner of Yukos Oil, and a former business partner, Platon A. Lebedev, are accused of laundering more than $20 billion and embezzling hundreds of millions of tons of oil at a time when Yukos was Russia’s largest private company. The charges carry a maximum sentence of more than 20 years in prison.

The bespectacled Mr. Khodorkovsky, his head shaved, arrived at the court amid a throng of police officers and journalists on Tuesday morning for preliminary hearings. It was the first time he had appeared in public since 2005, when he was convicted of fraud and tax evasion, given an eight-year sentence and sent to a Siberian prison.

His supporters roundly denounced that trial as a legal farce orchestrated by the Kremlin to eliminate Mr. Khodorkovsky, a potentially powerful opponent of Vladimir V. Putin, who was then Russia’s president. Mr. Khodorkovsky and his supporters have similarly accused Mr. Putin, now prime minister, and his allies of inventing the new charges to keep Mr. Khodorkovsky and Mr. Lebedev behind bars indefinitely.

Karinna Moskalenko, one of Mr. Khodorkovsky’s lawyers, called the charges against her client “absurd” and “groundless” and criticized the conduct of the first day’s proceedings, in which Judge Viktor Danilkin rejected a series of motions by the defense.

Defense lawyers sought to remove the two main prosecutors in the trial, who were also prosecutors in Mr. Khodorkovsky’s first trial, accusing them of bias. But Judge Danilkin indicated that they would remain in place.

The court also denied a defense request to free the defendants from the glass-and-steel box they will be enclosed in for the duration of the trial, a practice that is common at trials in Russia but that lawyers say contravenes international law.

Tuesday’s hearing was closed to the press, but in a statement posted on his Web site, Mr. Khodorkovsky accused prosecutors of trying to “complicate the essence of the case in order to confuse the court and society.”

He added, however, that he would “not speak about the politically motivated nature of the process in open hearings so as not to complicate the shocking simplicity of the case” — presumably alluding to the facts of the case itself.

The preliminary hearings are to continue Wednesday, but it is unclear when the actual trial, which court officials have promised will be open to journalists, will begin. Lawyers for Mr. Khodorkovsky said the trial could last as long as half a year.

The proceedings are taking place at the Khamovnichesky Regional Court, a crumbling structure of brick and stucco that is wedged between two luxury apartment complexes not far from the United States Embassy and Mr. Putin’s offices.

Riot police officers set up cordons blocking those without press or court credentials from approaching the building. Several dozen supporters of Mr. Khodorkovsky gathered in a small square a few blocks away for an unsanctioned protest. The police detained four who had broken into shouts of “Free Khodorkovsky,” according to witnesses.

Sergei Davidis, a member of the opposition movement Solidarity, praised Mr. Khodorkovsky for choosing to stay in Russia to fight several years ago, rather than leave when it became clear that he might be arrested. “In addition to being a symbol of repression, illegality and oppression of civil society,” Mr. Davidis said, “he is simply a living human being, who is behaving very bravely.”

The police seemed unfazed by some protesters, who wrote slogans in colored chalk on the pavement calling for Mr. Khodorkovsky’s release. One read, “Down With Legal Nihilism,” a slogan that Dmitri A. Medvedev voiced shortly after taking over as president last year.