Russian police arrested more than 300 people as they gathered in Moscow on Saturday to demand free and fair elections, a monitor said, following a crackdown on the opposition.
The rally comes a week after the capital's biggest demonstration in years, when some 22,000 people protested the authorities' decision to block opposition candidates from standing for the city council in September.
Investigators raided the homes and headquarters of several disqualified candidates in the run-up to the fresh rally on Saturday.
Top Kremlin critic Alexei Navalny was jailed for 30 days for calling for the demonstration.
Other leading opposition figures and would-be candidates were also arrested in the hours leading up to the event, which comes amid declining living standards and a fall in President Vladimir Putin's approval ratings.
OVD Info, which monitors protests, said at least 317 people had been arrested in the first hour of the demonstration.
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"Honestly, I'm scared," 42-year-old IT worker Alexei Sprizhitsky told AFP at the demonstration.
He said the last time he had seen this level of pressure on activists was in 2012, when Putin's return to the Kremlin after four years as prime minister sparked a wave of protests.
Local polls are a rare opportunity for dissenting voices to participate in political life as anti-Kremlin parties have been squeezed out of parliament over Putin's two decades in charge.
Security was tight in central Moscow and police shut down the area outside city hall where protesters were planning to gather, forcing participants out onto side streets.
"This is our city!", "Shame!" and "We want free elections," the crowd chanted as police blocked off the site.
Politician and disqualified candidate Dmitry Gudkov was arrested shortly before the march. Earlier he had said the future of the country was at stake.
"If we lose now, elections will cease to exist as a political instrument," he said.
"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."