KIROV, Russia -- The most prominent leader of the Russian opposition movement was sentenced to five years in prison for embezzlement on Thursday, a harsh sentence that seemingly reflected his rise as a political threat to President Vladimir V. Putin.
The opposition figure, Aleksei A. Navalny, who famously branded the president's United Russia political machine the "party of swindlers and thieves," had grown in stature from his beginnings as an anti-corruption blogger and leader of street protests. He had recently decided to run for mayor of Moscow, though he was given scant chance of winning.
Mr. Putin has shown a willingness to tolerate a certain amount of dissent, particularly on the Internet. But he has drawn the line at political challenges, as he did with the oil billionaire Mikhail Khodorkovsky, who was imprisoned in 2005 after he began backing independent political parties to challenge the Kremlin. He remains in jail.
Mr. Navalny has said he wants to be president one day, but he posed a different sort of threat, a steadily growing popularity combined with an incorruptibility that made him impossible to co-opt and a relentless effort to embarrass officials by disclosing their corrupt dealings.
As night fell in Moscow, thousands of protesters gathered in Manezh Square, a rare mass demonstration in the months since the government increased the penalties for unsanctioned gatherings. Lacking a permit, they were encircled by police in green camouflage, who quickly began making scattered arrests.
As the crowd swirled near Red Square, Dmitri Gudkov, a political opposition leader and member of Parliament who attended Mr. Navalny's sentencing, posted a message on Twitter saying that prosecutors had asked that Mr. Navalny be released, with travel restrictions, until his sentence takes effect, following a mandatory appeal.
"Tomorrow morning he may be released," Mr. Gudkov wrote. "Manezh, this is thanks to you!"
Mr. Navalny, a 37-year-old lawyer, had long dismissed the trial as a charade based on trumped-up charges, a contention backed by the United States and the European Union. He remained defiant to the end, spending much of the three-hour session in a local court here posting messages and photos on Twitter, ignoring an order from the judge, Sergei Blinov, to shut off all cellphones, and denouncing the evidence against him as "falsified."
Before being led out in handcuffs, he sent followers one last message: "O.K. Don't miss me. And most importantly -- do not be lazy." Referring to the Russian government, he added, "The toad will not remove itself from the oil pipeline."
If his conviction is upheld on appeal, Mr. Navalny will be prohibited for life from holding public office.
The case here in Kirov was the most high-profile of a series of politically charged prosecutions of Mr. Navalny and other opposition figures in recent months, as the Kremlin has demonstrated its willingness to use the judicial system for political retribution undeterred by criticism, at home or abroad. The verdict incited some calls for boycotts of the Moscow mayoral election and future national ballots, and drew cries of alarm from the West.
Depending on how history unfolds, the verdict may have ended Mr. Navalny's political career or sealed its future success. But the prospect of a lengthy jail term immediately catapulted him from opposition activist into the ranks of Russian dissidents, the first among that historic archetype to use the Internet and social media as his main weapon against the state. For many in Russia, the sentence seemed to mark a turning point.
"I think it's always hard to say which point is the bifurcation point, the threshold after which there is irreversibility," said Sergei Guriev, a public supporter of Mr. Navalny and prominent Russian economist who recently fled to France fearing his own political prosecution. "But this is certainly one of the big moves."
"The message is that whatever you do, even if you do socially useful things, if you are in opposition to the government, you are going to jail," Mr. Guriev said. He added, "Everyone in the government knows who Navalny is. Everyone in the government reads his blog. He is the face of the Russian opposition. He is the face of the younger generation. What happens to him determines the future of Russia."
While some supporters expressed surprise that the Kremlin would go so far as to jail Mr. Navalny, his sentencing fits a pattern that Mr. Putin has set since returning to the presidency, of harshly cracking down on dissent, limiting foreign influences and showing little regard for what the rest of the world thinks.
From the stiff sentences meted out last year to the members of the girl punk band Pussy Riot for their stunt protest in a Moscow cathedral, to his public musing about granting asylum to the fugitive former intelligence contractor Edward J. Snowden, Mr. Putin has made clear that Russia's values are its own and that the rest of the world will not tell him what to do.
The United States and the European Union condemned the verdict. "We are deeply disappointed in the conviction of Navalny and the apparent political motivations in this trial," the American ambassador in Russia, Michael A. McFaul, wrote in a Twitter message.
Catherine Ashton, the European Union's foreign policy chief, issued a statement saying the verdict "raises serious concerns as to the state of the rule of law in Russia."
Jailing Mr. Navalny poses special risks for the Kremlin. Handsome, with angular features and dimple in his square chin, Mr. Navalny holds political views that include a nationalist streak. His wife, Yulia, tall and blonde, exudes Russian beauty and they have two adorable children, a daughter, Dasha, 11, and son, Zakhar, 5.
Unlike Mr. Khodorkovsky, the former Yukos oil tycoon, Mr. Navalny does not carry the baggage of being a billionaire oligarch. He is strictly a political creature, whose main role has been in exposing public corruption and helping lead a series of large street protests in Moscow following the disputed parliamentary elections of December 2011.
As the five-year prison sentence was delivered, and the judge said that it could not be suspended but required actual jail time, some of Mr. Navalny's supporters burst into tears. He turned and hugged his wife, then shook hands with some supporters before being led away by guards. Mr. Navalny's elderly parents were also in the courtroom, and at one point bickered with bailiffs who complained that they were not standing up.
After the sentencing, Mr. Navalny's longtime press secretary, Anna Veduta, sat on a bench in the courthouse with tears streaming down her face. Ms. Navalny sat beside her, dry-eyed and stone-faced but slightly pale. They remained there for about 10 minutes before leaving through a back exit. Outside the court, Ms. Navalny said that her husband's work would not be halted.
"Aleksei was as ready for this as one can be," she said. "If anyone believes that Aleksei's investigations will cease, that is not the case. The Fund for the Fight Against Corruption will continue working as before."
While the guilty verdict was widely expected, the charges against Mr. Navalny, of stealing nearly $500,000 from a state-controlled timber company, were considered thin by many legal experts and had previously been thrown out as baseless after a local investigation. The case was resurrected by federal officials in Moscow and the Kremlin had made little effort to mask the political motivation of the prosecution.
A spokesman for the federal Investigative Committee, Vladimir Markin, declared publicly that Mr. Navalny had made himself a target through his political activities criticizing public officials.
"If a person tries with all his strength to attract attention, or if I can put it, teases authorities -- 'Look at me, I'm so good compared to everyone else' -- well, then interest in his past grows and the process of exposing him naturally speeds up," Mr. Markin said.
Mr. Navalny was convicted along with a co-defendant, Pyotr Ofitserov, a businessman and acquaintance who had worked with him on the timber project when Mr. Navalny worked as an unpaid adviser to the regional governor here. Mr. Ofitserov, a father of five who has not been politically active, was sentenced to four years in prison. The two men were also each fined more than $15,000.
The judge's findings were largely based on the testimony of a third man accused in the scheme, Vyacheslav Opalev, who pleaded guilty and worked with the prosecution.
In his decision, Judge Blinov called Mr. Opalev's testimony trustworthy and reliable. But during the trial, Mr. Opalev at times gave contradictory evidence, and defense lawyers were not allowed to cross-examine him. In addition, Judge Blinov barred the defense from calling 13 witnesses.
Unless the conviction is reversed on appeal, which seems unlikely, the verdict stood to disqualify Mr. Navalny from the Moscow mayoral election, which will be held in September. The incumbent, Sergei S. Sobyanin, was widely favored to win, even if Mr. Navalny had not been convicted. Still, many said the prosecution was partly intended to keep him off the ballot and to end his political career.
Aleksei L. Kudrin, a close associate of Mr. Putin and former finance minister, described it on Twitter as "looking less like a punishment than an attempt to isolate him from social life and the electoral process."
The crime novelist Boris Akunin, who is also a political opposition leader, said the verdict showed there was little hope to change Russia by democratic means.
"Lifetime deprivation of elections -- this is what the verdict means not only for Navalny but for all who thought it was possible to change this system through elections," Mr. Akunin wrote. "As long as the Putin regime is alive, there will not be elections. The answer to the question 'To be, or not to be' that is to boycott or not boycott, has been answered. For other elections as well."
Reporting was contributed from Moscow by Ellen Barry, Andrew E. Kramer, Andrew Roth, Alexandra Kozlova, Anna Tikhomirova and Noah Sneider.world
This article originally appeared in The New York Times.