Helmeted police in Moscow rounded up nearly 700 demonstrators and at least one opposition leader Saturday in an effort to squelch an unauthorized march protesting the exclusion of independent candidates from fall city council elections.

On Pushkin Square, a gathering point not far from the Kremlin where the protest march was to begin, police waded into a scattered crowd standing in light rain and detained dozens.

Officers quickly detained Lyubov Sobol, an opposition figure and one of the candidate hopefuls, as she was heading to the rally. She was taken away in a police van.

At least 685 people were detained, the independent monitoring group OVD-info reported, according to the independent Moscow Times reports. Police said they had detained 30 people

At another square along the planned 2.5-mile protest route along Moscow's Boulevard ring, helmeted riot police linked arms and swept people away.

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The protest was livestreamed on Russian independent websites.

Authorities had warned the unauthorized protest march through central Moscow Saturday would bring a strong police response.

The city prosecutor’s office on Friday issued a statement warning that the attempt would be “a direct violation of the law. Law enforcement agencies will be forced to take all necessary measures to curb provocations.”

It was the third Saturday in a row that demonstrators turned out to protest the exclusion of opposition candidates for election to the 45-seat Moscow city council, which is controlled by the pro-Kremlin United Russia.

Authorities have denied that the opposition-backed candidates were specifically excluded from the ballot, claiming that they failed to garner enough valid signatures. The opposition said the allegations were false.

Sergey Radchenko, a history professor, told Moscow Times city authorities fear Kremlin-backed candidates would lose in big numbers if independents were on the ballot in September.

"Russian regional elections last fall showed that the regime can lose even when it resorts to electoral fraud," he told the newspaper. "If seats are lost to the opposition — first at the local levels, then perhaps even at the national level — the Kremlin will be placed in an untenable situation."

Last week, at an unsanctioned protest outside the Moscow mayor's office, more than 1,000 demonstrators were violently detained by police. An independent organization that monitors arrests put the number of detained at 1,373.

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The initial demonstration, which had been officially approved, was held farther out from central Moscow.

Days before last week's demonstration, opposition figure Alexei Navalny was jailed for calling the protest. Also detained in advance were other prominent opposition politicians, including Ilya Yashin, Dmitry G. Gudkov and Ivan Zhdanov.

Navalny, a longtime political foe of Russian President Vladimir Putin, was later taken from jail and hospitalized, prompting fears that he had been poisoned.

Russia's Sklifosovsky toxicology institute said "poisoning substances were not found" in Navalny's samples, according to Russian news agencies. Navalny's personal physician, Anastasia Vasilyeva, blasted the official denying, saying it was "completely absurd" because bedsheets and clothes were not tested, Deutsche Welle reported.

The 43-year-old Navalny has called Putin's United Russia party a place of "crooks and thieves" and has accused the president's system of "sucking the blood out of Russia," the BBC reports.

Contributing: Associated Press