The Russian opposition vowed to stage another mass rally Saturday despite increasing pressure from authorities, who arrested nearly 1,400 people at a protest last week and have launched a criminal probe into the movement.
The unauthorised march along Moscow's leafy boulevards will be the latest in a series of demonstrations after officials refused to let popular opposition candidates run in next month's city parliament elections.
The local issue has boiled over into one of the worst political conflicts of recent years, with rallies of up to 22,000 people and police violence against demonstrators.
More than 6,000 people said on Facebook they would take part in the march along Moscow's so-called Boulevard Ring on Saturday to "bring back the right to elections".
Authorities installed barriers along the route ahead of the protest.
Candidates needed to collect signatures from city residents to stand in the polls but officials said they were disqualified because some names were forged.
But candidates, including allies of top Kremlin critic Alexei Navalny, insist their signatures were thrown out arbitrarily, and the whole vetting process was skewed against them.
Many Muscovites said their signatures in support of opposition candidates were declared invalid for no reason.
President Vladimir Putin has yet to comment on the situation in Moscow.
- 'Right to choose' -
In the polls in September, the opposition hopes to end the monopoly of Kremlin loyalists in Moscow's parliament.
The body decides the city's multi-billion-dollar budget but lacks political independence from mayor Sergei Sobyanin, an ally of Putin.
Sobyanin has warned the opposition against "new provocations".
Navalny and other protest leaders argue corruption is rife in the capital.
Navalny, currently in jail, was rushed to hospital last weekend in an incident his personal doctor blamed on possible poisoning with an unknown chemical substance.
A state toxicology lab said no traces were found.
Navalny's team on Thursday released a report accusing Sobyanin's deputy of selling prime Moscow property to her family members at rock-bottom prices.
The claims have angered protesters who have called for more street demonstrations.
"The Electoral Commission is not doing its job, the courts are not doing their job. The only thing that works, is the street," said one young woman on Twitter.
Authorities have launched investigations into last weekend's "mass riots" and "violence against police", echoing similar probes into protests against Putin in 2012 which saw several people jailed.
Four people were formally arrested pending trial Friday, though the opposition stressed that the protest was peaceful.
"Their task is to paralyse the protest movement, to isolate its leaders and frighten participants," one rejected candidate, Ilya Yashin, wrote on Facebook.
He is currently being held for 10 days in a police cell for violating rules on legal protests.
"There were no mass riots in Moscow, not by a long shot," Yashin wrote.
- Hunger strikes -
One of the men arrested is suspected of injuring a policeman by allegedly throwing a plastic bottle into the ranks of national guard wearing heavy protective gear.
Several candidates have declared hunger strikes.
Moscow police on Friday issued a warning for people not to attend the rally, which has not been given permission by the authorities to go ahead.
"We repeat that the event is illegal," it said on its website. "We suggest that residents and visitors refrain from participating."
Police "will take all necessary measures," the Moscow prosecutors' office said.
Reports said that the head of Moscow police even petitioned the Russian Premier League to reschedule a match between the city's two top teams, which may trigger fan violence and require a massive presence of the force in two places at once.
The petition was rejected, a source told TASS news agency.