The Russian opposition leader Alexei Navalny was jailed for 30 days on Monday on protest-related charges that he alleged were designed to prevent him organising demonstrations against unpopular government plans to raise the national retirement age.

Navalny, 42, was arrested on Saturday outside his home in Moscow, and charged with organising an unsanctioned opposition rally near Red Square on 28 January. His arrest came shortly after he had called on Russians to take to the streets on 9 September to protest against proposed pension reforms that would raise the retirement age from 55 to 63 for women and from 60 to 65 for men.

Police said they had not detained Navalnyover the January rally before Saturday because they had been unable to locate him. This despite the fact that Navalny spent much of May in police custody after being arrested at a protest in central Moscow on the eve of Vladimir Putin’s inauguration for a fourth term of office. “We both understand that the sole purpose of this strange trial is to keep me from participating in preparations for 9 September,” Navalny told the judge before sentencing.

Putin pledged in 2005 that he would never raise the state pension age, and his approval rating dropped from 77% to 63% after the government’s plans were announced in June, according to VTsIOM, the state-run pollster. About 90% of voters are opposed to the move, according to opinion polls.

The plans to reform the pension system have sparked widespread anger because life expectancy in Russia remains low, at 66 for men and 73 for women. The Kremlin, however, saysRussia’s ageing population means Putin had no alternative but to go back on his word.

An anti-corruption activist who has alarmed some members of the Russian opposition with his ties to the country’s nationalist movement, Navalny also said on Monday that the justice ministry had rejected his application to register a political party called “Russia of the Future.”

It is the third time in six years that Navalny and his supporters have tried unsuccessfully to register a party that could stand in regional and national elections. He was also barred from challenging Putin at this year’s presidential election over a previous conviction for fraud that he said the Kremlin had trumped up to stifle his political ambitions.

“Mass protests are the only way to influence the authorities. Putin has cut off all the others,” Navalny said in a video posted online ahead of his jail sentence.