“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: “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.”

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 oppose the Kremlin. He remains in prison.

Mr. Navalny, 37, a 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, spending much of the three-hour session in a local court here posting messages and photographs on Twitter, ignoring an order from Judge Sergei Blinov to shut off all cellphones and denouncing the evidence against him as “falsified.”

Before being led out of court, he sent followers one last message: “O.K. Don’t miss me. And most importantly — do not be lazy.”

If his conviction is upheld on appeal, Mr. Navalny will be banned from public office for life. He 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 relentlessness in embarrassing officials by disclosing their corrupt dealings.

The case here in Kirov was the most high-profile in a series of politically charged prosecutions of Mr. Navalny and other opposition figures in recent months, as the Kremlin has shown 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. While the verdict could eventually prove either to end Mr. Navalny’s political career or seal its future success, for many in Russia it seemed to be a turning point.