Police in Moscow arrested scores of people on Saturday during an unauthorized rally to demand fair upcoming elections in Russia's capital city.
OVD-Info, a monitoring group, said that at least 500 people had been detained in central Moscow by Saturday afternoon.
Demonstrators are protesting a decision by Moscow's electoral authorities to keep several opposition members off the ballot for a city council election in September. The opposition candidates allegedly used fake signatures on their nominating petitions.
Alexei Navalny, Russia's most prominent opposition activist, had called for people to attend the rally. He was jailed for 30 days on Wednesday for organizing "unauthorized" demonstrations.
Officials in the Russian capital have declared the demonstration illegal. Moscow Mayor Sergei Sobaynin said the protests were a "security threat" and vowed that "order will be ensured according to the law."
Two of Navalny's associates, Ivan Zhdanov and Ilya Yashin, said they had been detained ahead of the demonstration.
Navalny has said protests will continue until the city allows the rejected candidates to run in the election.
Read more: Russians rally for independent candidates to stand in elections
Moscow police block demonstrators in front of Moscow city hall on Saturday
Investigators reportedly raided the homes and headquarters of several disqualified candidates in the run-up to the protest.
Dmitry Gudkov, a disqualified opposition candidate, told the AFP news agency that Russia's future was at stake.
"If we lose now, elections will cease to exist as a political instrument," said Gudkov, whose home was raided Wednesday.
"What we're talking about is whether it's legal to participate in politics today in Russia, we're talking about the country we're going to live in."
Last weekend, 22,000 people attended a protest in Moscow to protest the ban on opposition candidates, the largest such demonstration in years.
The opposition sees the local elections as an opportunity for political participation, as anti-Kremlin parties have been steadily marginalized in Russian politics since President Vladimir Putin first took office 20 years ago.
Kremlin-backed candidates have proven less popular in the capital than in other parts of Russia. However, independent parties say they face many bureaucratic hurdles in running for office compared to pro-Kremlin candidates.
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