Russian authorities have threatened protesters in Moscow with lengthy jail sentences in an attempt to dampen an unexpected surge in protest mood before a planned rally on Saturday.

Last weekend police detained a record number of people, some of them violently, for taking part in a peaceful protest in central Moscow over access for opposition candidates to local elections in September.

Most of the more than 1,300 people detained were released immediately, but many of the candidates remain in jail, and the Russian news agency Tass reported that at least nine people were arrested on charges of organising or participating in “mass unrest” on Friday, which can carry a jail term of up to 15 years. Three people appeared in court on Friday and a judge ordered they be held in jail for two months while the investigation continued.

Almost all of the violence last weekend came from police. The worst act by a protester captured on camera was a man throwing a rubbish bin in the direction of riot police. However, Moscow’s mayor, Sergei Sobyanin, made it clear authorities would take a hard line when he appeared on television earlier this week to thank police for carrying out their duties and claimed they had foiled “pre-planned and well-prepared mass unrest”.

Elections to Moscow’s city parliament were expected to pass off without much interest, but there has been a furious response to the refusal by the authorities to let independent candidates stand. One of them, Lyubov Sobol, is three weeks into a hunger strike over her demands to be allowed to stand.

The opposition politician Lyubov Sobol, who is on hunger strike to protest against her exclusion from Moscow city council elections, is led away after an interview with Reuters this week. Photograph: Tatyana Makeyeva/Reuters

“There is not a single reason that we should not be allowed to take part in the elections,” she told the Guardian in an interview earlier this week.

Opposition figures have called for a protest march on Saturday along Moscow’s central boulevard ring. Authorities suggested an alternative venue, further from the centre, but this was rejected by protest leaders, meaning the rally is “unsanctioned” and there is the prospect of further mass arrests.

Alexei Navalny, the most prominent opposition leader, was arrested before last weekend’s rally and jailed for 30 days. On Sunday, he was rushed from prison to hospital with swelling and a rash which his personal doctors said indicated potential poisoning.

He was returned to prison on Monday, against the wishes of his doctors, after the hospital where he was treated announced to press that there was no sign he had been poisoned and instead had hives.

Vladimir Putin has not yet commented on the protests, but the official response is reminiscent of the last big wave of protests in Russia, in 2011 and 2012. They culminated in a huge rally the day before the president was inaugurated in May 2012. Several protesters were put on trial and given long sentences.