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.
"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.
- 'We want free elections' -
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."
While pro-Kremlin candidates enjoy the support of the state, independent candidates say they have been made to jump through countless hoops in order to get on the ballot for the city polls.
After activists and ordinary Muscovites staged pickets last week, including outside the local election commission building, investigators said they were launching a criminal probe into obstructing the work of election officials.
If found guilty, organisers risk up to five years in prison.
- 'Afraid all my life' -
Navalny ally Ivan Zhdanov said he had been arrested shortly ahead of the demonstration. Barred candidate Ilya Yashin meanwhile announced he was detained in the early hours of Saturday morning following a raid on his home.
Would-be candidate Lyubov Sobol, who this week launched a hunger strike, was arrested at the demonstration.
Moscow Mayor Sergei Sobyanin called the unauthorised protest a "security threat", adding that "order will be ensured according to the relevant laws".
Elena Rastovka, a 68-year-old pensioner at the demonstration, told AFP: "I've been afraid all my life, but enough is enough. If we stay at home, nothing will change.
"Authorities arrest people who want to challenge them. Look at what they're doing -- the authorities do not like the people."
Some said it was the authorities' heavy response that had turned a local issue into a major protest movement.
"Who would have thought it would become important to take part in such a bizarre and boring affair as the Moscow parliament election?" asked Viktoria Popova, a 30-year-old illustrator, ahead of the rally.
Nearly 11,000 people indicated interest in the rally on Facebook.
Police asked media to notify the authorities if they planned to cover the protest and urged Russians to skip the rally altogether.