Heeding a call from opposition leader Navalny, thousands took to the streets of dozens of cities against the controversial pension reform.
Navalny, who ran against Sobyanin in 2013 and had planned to lead a protest against proposed pension changes on Sunday, will spend election day behind bars after being convicted of violating protest laws. An Associated Press journalist counted at least 30 people detained at that protest.
More than 1,000 people were detained at anti-government protests across the country in what the Kremlin on Monday called a legitimate response to unauthorized rallies.
A further 180 protesters were reported to have been detained in the city of Yekaterinburg, in the Ural Mountains, 60 in the southern Russian city of Krasnodar, 40 in the Siberian city of Omsk, and 40 in Russia's capital, Moscow.
Almost half of those detained were rounded up in St. Petersburg, according to the OVD-Info.
Mr Putin softened the initial plan to raise women's retirement age to 63.
The proposed pension overhaul, which is now before parliament, has sparked a rare outburst of public anger that has seen President Vladimir Putin's approval ratings plummet. Photos and video footage from St. Petersburg show the police violently beating protesters.
But opposition leader Alexei Navalny, who is serving a month-long jail term over an earlier illegal protest, is trying to spoil the party with nationwide demonstrations against pension reforms.
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Communists beat candidates of the ruling party in elections to the regional parliaments of Khakasia, Irkutsk and Ulyanovsk regions.
They are the most unpopular government measure since a 2005 move to scrap Soviet-era benefits, though Putin amended the planned reforms after initial protests.
Moscow Mayor Sergei Sobyanin has voted at the Moscow mayoral election on Sunday morning at a polling station at his place of residence in central Moscow.
Officers held dozens more as protests kicked off in eastern Russian Federation and Siberia, with demonstrations set to take place in Saint Petersburg and Moscow in the afternoon.
A lawyer for Navalny's Anti-Corruption Fund was arrested in Moscow before the rally. Last month he offered some concessions, but he and government officials said the age hike is necessary because rising life expectancy in Russian Federation could exhaust pension resources if the eligibility age remains the same.
The scale of the electoral setbacks for United Russia is "a unique situation", said Dmitry Orlov, a political analyst who's a member of the party's supreme council.
Olga Sokolova, a 52-year-old factory worker, said she was "dumbfounded" when the proposal came because she had hoped to retire from her physically taxing job at 55.