The homes of several opposition politicians have been raided by Russian police ahead of an unauthorised rally demanding fair elections.
Moscow officials barred opposition candidates from running in 8 September local elections, then arrested key Kremlin critic Alexei Navalny ahead of Saturday's planned mass protest.
Police then raided four candidates' homes, calling in more for questioning.
Critics said the crackdown proved how nervous authorities were of the rally.
One of the council election candidates whose home was searched, Dmitry Gudkov, complained that the raids meant the elections were over. "This institution has died under Putin. The last illusion that we are able to participate legally in politics has disappeared," he tweeted.
Why has the row escalated?
Earlier this month, Moscow city authorities turned down dozens of opposition candidates, arguing that some of the required 5,500 signatures they had collected were invalid, ahead of the city council vote.
The local authority is held by United Russia, a party with close links to the Kremlin, and control of the council is seen as symbolically significant in Russia.
Then a rally on 20 July brought more than 20,000 people on to the streets. Thousands more indicated they would take part in the next big rally, but authorities refused to allow it, saying that the protests included threats of violence against the electoral commission.
Mr Navalny has been jailed for 30 days and a colleague has been detained for eight days.
Condemning the "hysterical actions" of the authorities, ex-mayor of Yekaterinburg Yevgeny Roizman said it was easy to put a stop to the rally. "Just let all the independent candidates who have collected signatures run," said Mr Roizman, who stood down when mayoral elections were scrapped in Russia's fourth largest city last year.
Why authorities fear dissent
By Oleg Boldyrev, BBC News, Moscow
For almost three weeks protests around Moscow elections went unhindered. But then last weekend's rally attracted around 20,000 people.
Perhaps, the authorities hoped one, big gathering would be enough for most Muscovites. But both unregistered candidates and their supporters have vowed to carry on, and having a sustained campaign in the centre of Moscow is not the mayor's idea of a good summer,
In fact, many analysts believe control of events has now switched from Moscow to the Kremlin. The would-be candidates are in no doubt the authorities want them off the streets but say the rally will happen no matter what.
Experience dictates that the protest will be dispersed, most likely with heavy force, detentions, speedy court hearings and heavy fines. It is rare for momentum to build up in the face of a crackdown, but this is what the opposition is hoping for.
One thing is certain, a dull local election campaign for has become a focus for dissent, not just over the way things are done in Moscow but over the lack of political competition in Russia as a whole.