Human rights activists in Russia say that the convictions of two leftist leaders, Sergei Udaltsov and Leonid Razvozzhayev, are a clear signal that nothing is changing for the better in Vladimir Putin's Russia.

Mr. Udaltsov and Mr. Razvozzhayev are now on hunger strike after being sentenced to 4-1/2 years in prison for staging "mass disorders" aimed at overthrowing Mr. Putin on the eve of his inauguration to a third presidential term in the Kremlin just over two years ago.

Activists say the trial was a travesty of legal procedures, part of the Kremlin's systematic crackdown on civil society and opposition activities over the past couple of years.

"The arguments against the defendants were totally far-fetched. Basically they were convicted because they were activists in opposition, and took part in a peaceful rally," says Lyudmila Alexeyeva, a founder of the Moscow Helsinki Group, Russia's oldest human rights monitor. "The authorities are out to make examples of anyone who takes part in civil activism."

Udaltsov, leader of the neo-Bolshevik Left Front, was accused of being at the center of a bizarre revolutionary conspiracy involving foreign cash and a Georgian secret agent. His co-defendant, Mr. Razvozzhayev, was allegedly "renditioned" by Russian secret services from a Kiev street as he was trying to seek political asylum, and secretly spirited back to Russia.

The actual event for which the two men were prosecuted was a fairly minor scuffle [witnessed by this correspondent] on the fringes of an otherwise peaceful anti-Kremlin rally of about 20,000 people in downtown Moscow's Bolotnaya Square on May 6, 2012. A few dozen protesters appeared to attack massed ranks of armored riot police, throwing bottles and paving stones, and police responded with a baton charge and large-scale arrests. Most participants in the rally seemed unaware that anything had happened.

That bout of disorder, which probably would have gone unremarked had it happened in Paris or London, sparked a massive two-year investigation and wave of trials that has become known as the "Bolotnaya Affair." A special police task force of 200 officers was created to investigate the causes of the riot, and dozens of arrests were subsequently made.

The pro-government NTV network produced a documentary film, using surveillance tapes critics say could only have been provided by Russian secret services, to depict Udaltsov as a revolutionary conspirator seeking advice from a Georgian operative and cash from exiled anti-Kremlin businessmen.

Critics say it was never established how this elaborate alleged conspiracy ended up in the melee on Bolotnaya Square, which neither Udaltsov nor Razvozzhayev took any direct part in.

"I witnessed the trial, and was stunned by the verdict. It had nothing to do with the law," says Sergei Davidis, a human rights lawyer who works with the Solidarnost opposition coalition.

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"There was no solid evidence of anything they were accused of. It was said they met with a Georgian operative, who promised them money, but the only witness for this was [a defendant who turned state's evidence and was released on parole]. Otherwise they are blamed for organizing the rally, but that was perfectly legal political activity." Mr. Davidis says the main purpose of the trial was to create a "court-supported" narrative for public consumption, to show that people who demonstrate in the streets are pawns of foreign interests and paid provocateurs.

"I'm not so shocked at the four-and-a-half year sentence they got. That's not so bad [in Russian terms]," he says. "But the complete mockery of justice and blatant illegality on display in this trial was something truly appalling."