A Russian opposition leader has been found guilty of organising mass rioting at a protest on the eve of Vladimir Putin's return to the Kremlin in 2012, in a trial that human rights experts describe as politically motivated.

Sergei Udaltsov, leader of the socialist Left Front and a major voice in the wave of street protests that shook Russia in 2011-13, was convicted of planning the unrest at the protest, along with Leonid Razvozzhayev, an activist and aide to opposition MP Ilya Ponomaryov, the only deputy to vote against Russia's annexation of Crimea.

Udaltsov and Razvozzhayev were each sentenced to four-and-a-half years in a prison colony, while the latter was additionally fined 150,000 roubles for illegally crossing the border. Prosecutors had sought a maximum tariff of eight years. The defence plans to appeal.

The two men were among 28 people charged with rioting-related charges after a protest on 6 May 2012, when thousands of protesters from across the political spectrum gathered at Bolotnaya Square in Moscow on the eve of Putin's return to the presidency.

The rally ended in bloodshed after police in riot gear attempted to break it up, sparking clashes with angry protestors.

Video footage from the event shows members of both sides participating in the violence, and 70 police officers were allegedly injured. But the independent newspaper Novaya Gazeta reported that protesters suffered more serious injuries on the whole, including broken arms, ribs and noses. No criminal cases have been opened against any law enforcement officer.

Of the 28 protesters charged in the case, 12 have been given prison sentences ranging from 30 months to four-and-a-half years, including one suspended sentence. Several have been declared prisoners of conscience by Amnesty International, including Mikhail Kosenko, who was declared insane and sentenced to indefinite compulsory treatment in a psychiatric institution. Charges against 11 defendants were dropped after a wide-reaching amnesty law was passed this year.

Russia's powerful investigative committee has said it opened the case against Udaltsov and Razvozzhayev on the basis of footage from the film Anatomy of a Protest 2, which has been disparaged as an attack on the opposition. The grainy hidden-camera footage appears to show the activists meeting a man identified as Georgian politician Givi Targamadze, who promises to discuss transferring large sums of money to them. Udaltsov testified he met Targamadze to discuss a possible liquor importing business.

Tanya Lokshina of Human Rights Watch called Udaltsov and Razvozzhayev's trial and the rest of the Bolotnaya case a "mockery of justice", noting that an international panel of experts on free assembly, including members of the Organisation for Security and Co-operation in Europe, had found there were minor clashes with police but not the major riots the prosecution claims.

"This trial is meant to intimidate protesters in Russia," Lokshina said. "The sentences people have been getting, the fact they spent so much time in custody before trial, it's meant [to show] that the price of taking to the streets is very high."

Maria Baronova, a Bolotnaya defendant who was pardoned under the amnesty law, said the trial had contributed to the smothering of the opposition movement along with the numerous cases against opposition leader Alexei Navalny, who is being sued for defamation by a deputy mayor.

"Nobody wants to be involved in politics since it's forbidden to be involved in it," she said. "Each person knows he will lose something after participating in something like that."