Police detained Alexei Navalny, the Russian opposition leader, outside his home in Moscow on Saturday, in a move that his supporters say was aimed at preventing him from leading nationwide protests against unpopular government plans to increase the national retirement age.

His spokeswoman, Kira Yarmysh, said the 42-year-old Kremlin critic was treated briefly in hospital for a suspected broken finger. It’s thought he was injured during his arrest, which police said was linked to his organisation of an opposition rally in Moscow on 28 January. He will remain in custody before he appears before a court on Monday.

Supporters of Navalny alleged that the real reason for his arrest was his call for Russians to take to the streets on 9 September to protest against proposed legislation that would raise the age at which citizens can claim a state pension from 60 to 65 for men and from 55 to 63 for women. The average monthly pension in Russia is 13,342 roubles (£154).

“Navalny, who was heading to see his family in the countryside, in shorts and slippers, was detained by five riot police officers,” Yarmysh wrote on Twitter. “And all of this on the pretext of the 28 January protest. Just to hinder him ahead of 9 September. Can you imagine more cowardly authorities?”

President Vladimir Putin pledged in 2005 that he would never increase the state pension age, and the government’s plans have sparked widespread anger. Putin’s approval ratings fell from 77% to 63% in the two weeks after the pension reform plans were announced, according to VTsIOM, the state-run pollster.

A recent opinion poll by the Levada Centre, an independent pollster based in Moscow, indicated that about 90% of Russians opposed the plans, while 40% were ready to take part in street protests against them.

On 3 August, an explosive device partly destroyed the entrance to the offices of the national pension fund authority in Kaluga, a city in western Russia. No one was injured in the early morning blast.

Authorities in Moscow routinely turn down applications by Navalny and other opposition figures to hold protests in the centre of the Russian capital. Officials representing the mayor of Moscow, Sergei Sobyanin, said last week that they would not consider an application from Navalny to stage a rally near the Kremlin on 9 September.

“We don’t give a damn about the mayor,” a defiant Navalny said. “We need to take to the streets.”

Navalny has been jailed several times for organising unsanctioned anti-Kremlin protests, most recently in May, when he was sentenced to 30 days after leading a demonstration in Moscow on the eve of Putin’s inauguration for a fourth presidential term.

Putin’s ruling United Russia party introduced legislation to increase the national retirement age on 14 June, the opening day of the World Cup, triggering allegations that it was trying to bury bad news. The legislation would usher in the first changes to policy established under the Soviet dictator Joseph Stalin.

Russia has one of the lowest retirement ages in the world. The reforms are unpopular, however, because many Russians fear they will not live to claim their pensions. While life expectancies are rising, they remain low, especially for men, whose average age at death is only 66.

The Kremlin admits that Putin went back on his promise never to increase the retirement age, but says an ageing population left him with no alternative. By 2044, the numbers of pensioners could approach the number of people in work, according to government forecasts, putting pressure on the national budget.

Russia’s prime minister, Dmitry Medvedev, said the reforms were “unavoidable and long overdue”. Russian media reported last week, citing government sources, that Putin might announce a scaling-down of the proposed reforms on Tuesday, but the Kremlin has not commented.