A prison stint in most countries is a disgrace to live down, but in Russia it can be a valuable asset, even a must, on the resume of anyone who aspires to lead an anti-Kremlin protest movement.

Russian security services have always been ready to oblige their worst opponents by tossing them into jail on almost any flimsy pretext, with little regard for legal niceties or human rights conventions.

A century ago most Bolshevik leaders spent time in Czarist prisons, a rite-of-passage that burnished their revolutionary credentials. The Soviet Union attracted the outrage of the world for its treatment of dissidents such as Anatoly Sharansky, Vladimir Bukovsky, and Nobel Prize-winning physicist Andrei Sakharov.

Though the dissident movement was destroyed by KGB repression, most of its leaders forced to emigrate, Mr. Sakharov was ultimately released from his internal exile by reforming Communist leader Mikhail Gorbachev. Sakharov ended his days as a member of the first Soviet parliament and a celebrated symbol of national conscience.

"This happens in repressive regimes, in which the government is seen as a force that's opposed to the people rather than a force that represents them," says Masha Lipman, editor of the Moscow Carnegie Center's Pro et Contra journal. "In today's Russia, Vladimir Putin's whole period of leadership has been about enforcing his dominance over society."

As a new protest movement swells the streets, Russian authorities, true to form, have been boosting the street cred of several opponents by jailing them for taking part in "unsanctioned" meetings, including former deputy prime minister Boris Nemtsov and radical blogger Alexei Navalny, who suggested Monday that he might be ready to challenge Vladimir Putin if there were fair elections.

"When we get a chance to take part in elections, I am ready to fight for leading positions, including in the presidential vote," Mr. Navalny told journalists.

But the most dramatic case may be that of Sergei Udaltsov, a veteran left-wing street activist who's been arrested more than 100 times in the past five years. He was preemptively jailed on Dec. 4 on a variety of pretexts that seem so outrageous he has begun to attract support far beyond his own small, radical left-wing constituency.

Almost 2,000 people, including many liberals who've only recently learned Mr. Udaltsov's name, have signed a Facebook pledge to attend an unsanctioned rally Thursday in Moscow's downtown Pushkin Square to accuse the Russian authorities of holding Udaltsov as a political prisoner and demand that he be released.

"Udaltsov's arrests look like nothing more than a witch hunt," says Yevgeny Ikhlov, an expert with the Moscow-based grassroots movement For Human Rights. "There has been a whole chain of completely fabricated cases against him. There is no doubt that he's being held to keep him out of the protest movement and to prevent him from putting himself forward as a candidate in the coming presidential election."

Udaltsov was picked up for "jaywalking" on the morning of Dec. 4, Duma election day. When his five-day prison term expired, he was immediately rearrested on charges of disobeying police officers during a previous arrest in October. When he went on a hunger strike, he was hospitalized under heavy police guard. On Tuesday, a Moscow judge postponed his appeal for ten days, requiring him to remain in custody during that time. His next hearing is scheduled for Jan. 7, which is Christmas Day in Russia, suggesting that further postponement is likely.

"All this makes a total mockery of the law, of due legal procedure, and even common decency," says Violetta Volkova, one of Udaltsov's lawyers. "I've been practicing law a long time in Russia, and I know there are honest judges and there can be fair trials. But everything that's happening to Udaltsov seems to go by a different set of rules. I think this is only because they fear that he could be a strong opposition leader, so they are keeping him off the streets by any means possible."

Udaltsov, reached on his cell phone in a prison hospital Tuesday, says he believes himself completely innocent and hopes that public pressure will force the the authorities to change their decisions.

"They may not realize it, but our authorities are setting the stage for a revolutionary situation by their own behavior," Udaltsov says. "My case is intended as an object lesson to the others, to show how they will treat the most radical opposition leaders. They hope to split the opposition by instilling fear in some, making separate promises to others. They are masters of such games. But we should keep together. This all just goes to show that our state needs capital repairs, not just a few cosmetic changes."

Even Mikhail Prokhorov, the billionaire metals tycoon and owner of the New Jersey Nets basketball team who recently announced that he will run in the upcoming presidential election as a liberal alternative to Putin, says he will work to see that political prisoners such as former oil magnate Mikhail Khodorkovsky and Udaltsov are freed.

In a YouTube address to the Russian people posted Wednesday, Mr. Prokhorov said: "I take it as a personal tragedy that some of our citizens will be greeting the New Year in prison. I’m talking about Mikhail Khodorkovsky, Platon Lebedev, Sergei Udaltsov, and many other innocent prisoners.… I am especially sad that I cannot help them now. But when the time comes, I will help."