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The changes, going through parliament, have shaved around 15 percentage points off President Vladimir Putin's popularity and are the most unpopular government measure since a 2005 move to scrap Soviet-eras benefits, which led to nationwide pensioner protests.
Navalny, barred from state TV and prevented from running against Putin for president earlier this year, hopes to tap into public anger over the reform.
He had planned to lead a protest in Moscow on Sunday, but a court last month convicted him of breaking protest laws and jailed him for 30 days. Navalny said the move was designed to derail the protests.
His supporters are pressing ahead without him however, and holding rallies in more than 80 towns and cities, including Moscow and St Petersburg.
The first rallies took place in eastern Russia on Sunday morning. Footage of a rally in Ulan-Ude, some 4,400 km (2734 miles) east of Moscow, showed protesters walking through the city holding red balloons escorted by the police.
"Putin and his government have plundered the budget for the past 18 years," Navalny's team said in a pre-protest statement.
"All that time they assured us there would not in any circumstance be a rise in the pension age. And now they are putting it up. The authorities are not listening to people and that means it's time to take to the streets."
In Moscow the authorities rejected an application from Navalny's supporters to protest in the city centre, raising the possibility that the police may disperse the rally by detaining people, as they have often done in the past.
After being amended by Putin, the reforms envisage raising the retirement age for men to 65 from 60 and to 60 from 55 for women. Average life expectancy for men is 66 and for women 77. Opinion polls put Navalny's support in the single digits, but backers note he won almost a third of the vote in a 2013 Moscow mayoral race, and believe he could give Putin a run for his money if ever allowed to run against him on a level playing field.
Putin makes a point of never mentioning Navalny by name but has suggested he is Washington's pick for the Russian presidency.
Navalny has likened Putin to an autocratic tsar who has clung to power for too long. The authorities have not registered his Russia of the Future Party.
Elections to select the heads of 26 of Russia's 85 regions are also being held on Sunday, including in Moscow.
Google removed a YouTube advert by Navalny after authorities complained that the videos would violate an election silence law before Sunday's vote for regional governors, an aide to Navalny said on Saturday.