Russian police arrested almost 1,400 people as they gathered in Moscow at the weekend to demand free and fair elections, a monitor said, the biggest crackdown on a rally in the country in years.
Some 3,500 people took part in the unauthorised protest on Saturday, according to official figures, after authorities blocked prominent opposition candidates from taking part in city elections.
Police used batons on protesters as they tried to gather outside city hall, and AFP reporters at the scene saw demonstrators with injuries.
The rally comes amid wider public frustration over declining living standards that has hit President Vladimir Putin's approval ratings.
A week before, 22,000 took to the streets in a sanctioned protest, calling on authorities to reverse their decision ahead of the September city council vote.
After that demonstration, investigators raided the homes and headquarters of a number of disqualified candidates. Top Kremlin critic Alexei Navalny was jailed for 30 days for calling the fresh protest.
Navalny on Sunday was taken to hospital from jail after suffering what his press secretary said was a "severe allergic reaction", despite having never suffered from allergies.
Leonid Volkov, an ally of Navalny, said he had a similar allergic reaction after serving time last month in the same jail. But he suggested hygiene conditions rather than a "conspiracy" was to blame.
Several would-be candidates were detained before or during the protest on Saturday.
OVD-Info, an organisation that monitors protests, reported Sunday that 1,373 people were arrested.
It said this was the highest number since mass demonstrations in 2012, when tens of thousands protested Putin's return to the Kremlin after four years as prime minister.
Opposition activists have called for another rally for open elections next weekend.
'Disproportionate police force'
The US embassy in Moscow denounced the use of what it said was "disproportionate police force" against peaceful protestors.
The violence and arrests "undermine rights of citizens to participate in the democratic process," embassy spokeswoman Andrea Kalan wrote in a tweet.
The EU said police actions undermined "the fundamental freedoms of expression, association and assembly," while Amnesty International also criticised the crackdown.
Elections to Moscow's 45-seat legislative body, currently controlled by the pro-Kremlin United Russia party, are due to be held in six weeks.
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.
Following 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.
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 power.