Russia’s most effective anti-corruption campaigner and opposition leader was found guilty of embezzlement Thursday and sentenced to five years in prison in a verdict that sent shock waves throughout the country.

The conviction of Alexei Navalny, 37, a leading critic of President Vladi­mir Putin with a penchant for exposés and cutting jibes, brought condemnation from those who view the case as politically driven. Opponents said it showed that Putin recognizes few bounds in his determination to crush challengers.

Michael McFaul, the U.S. ambassador to Russia, posted on Twitter: “We are deeply disappointed in the conviction of Navalny and the apparent political motivations in this trial.”

A statement released by the office of the European Union’s foreign affairs chief, Catherine Ashton, said, “This outcome, given the procedural shortcomings, raises serious questions as to the state of the rule of law in Russia.”

Even before the conviction, U.S.-Russian relations had turned increasingly brittle. White House officials said Thursday that President Obama’s scheduled September trip to Moscow to meet with Putin is in limbo because of uncertainty surrounding Edward Snowden, the fugitive National Security Agency contractor who fled to Russia and is seeking asylum there.

Russia's president cracks down on dissent.

The conviction and sentencing of Navalny were based on old charges that Russian prosecutors had previously declined to pursue. The outcome ignited an unsanctioned rally in the center of Moscow, where by evening a crowd of several thousand had gathered along the sidewalks of streets leading toward Manezh Square.

Navalny’s case has been the most prominent in a series of criminal prosecutions that Russian authorities have launched against their critics since the outbreak of political protest in late 2011 — much of it led by Navalny.

The anti-corruption blogger had emerged at the head of an opposition movement that represented the sharpest challenge in years to the hold that Putin and his allies have maintained over Russian politics.

But Navalny and others quickly became the targets of a serious crackdown that Putin launched after he returned to the presidency in May 2012, with demonstrators arrested, laws passed to limit protests and some nongovernmental organizations required to register as foreign agents.

Even before his trial began in April in Kirov, a city about 550 miles northeast of Moscow, Navalny said he expected to be convicted on what he and his supporters contended were trumped-up charges. But Navalny had been hoping for a suspended sentence, in the belief that the Kremlin would want to avoid a backlash if it appeared too harsh.

As he was led into custody, it became clear that those in power in Russia have chosen not to be subtle as they crack down on the opposition.

“I think the protest is going to a different level,” said Denis Maslov, a 31-year-old auditor who turned out for the Moscow rally. “People who are willing to change Russia for the better end up in jail. That’s why I’m here.”

“They’re afraid,” said Natalia Novikova, 48, speaking of the authorities. “At least when Navalny was around, there was hope.” With passing cars honking in support, the police presence was heavy, and as many as 60 people were detained.

Navalny’s supporters had called his prosecution a decisive moment for the protest movement. In an unexpected move, prosecutors asked for Navalny to be placed under house arrest until his sentence goes into effect in 10 days — also the appeal period — perhaps in an attempt to allow public anger to dissipate. The judge made no public comment on the request, however, and Navalny remained in jail as of Thursday night.

Besides the protest in Moscow, unsanctioned rallies were held Thursday evening in St. Petersburg, Tomsk, Samara, Kazan and Izhevsk.

In a recent interview, Navalny had warned that “Putin will resist to the end” those seeking his ouster. “He understands if he leaves, he will go to jail,” Navalny said of the Russian leader.

Condemnation, defiance

Human rights advocates denounced the sentence. Former Soviet president Mikhail Gorbachev, in a message on his foundation’s Web site, called the prosecution and verdict “very depressing.”

Sen. John McCain (R-Ariz.) called the case a show trial. And Sen. Benjamin L. Cardin (D-Md.) called for Navalny’s immediate release.

“Today’s political conviction of Alexei Navalny continues Russia’s turn back to the darker days of the Soviet Union when courts were weapons of tyranny rather than legitimate defenders of the rule of law,” he said.

After Navalny was led out of the packed courtroom, his wife, Yulia, addressed supporters and journalists, striking a defiant tone.

“If someone hopes that his investigation will stop, it won’t,” she said. “The anti-corruption foundation will continue its work. We will win. And, please, believe that everything will be great.”

Navalny is registered as a candidate in Moscow’s September mayoral election. His conviction will not necessarily bar him from running, as long as his appeals have not been exhausted. Authorities in the capital have been eager to have him in the race, confident that he would lose to the acting mayor, Sergei Sobyanin.

Basis of charge

Navalny was charged with theft over a deal he arranged in 2009, when he was an assistant to the governor in Kirov — he found a middleman to buy timber from a struggling logging company. Because the middleman made a profit on the timber, the prosecution charged that Navalny had effectively deprived the lumber company of nearly $500,000 by getting it to agree to sell below market value.

If any business deal can lead to prison, Mikhail Prokhorov, the billionaire owner of the Brooklyn Nets and a former presidential candidate, wrote on Facebook, “I wonder how many talented young businessmen and lawyers right now, right here at this point, are mentally packing their bags?”

Yevgenia Albats, editor of New Times magazine, tweeted that the court had overturned capitalism.

In contrast to what has happened to Navalny, the former head of a Moscow district who was convicted of embezzling $42 million was given five years of probation by a judge on July 7.

Late Tuesday, Navalny posted his latest exposé— a story about Vladi­mir Yakunin, the head of the government-owned Russian Railways. Navalny alleged that Yakunin has funneled millions of dollars in public money into companies run by his wife and sons, many of them based offshore.