An unsanctioned protest organized by the Russian opposition has ended in clashes between the police and demonstrators, who pelted officers with stones. Tensions in the city center led to hundreds of arrests.

A standoff quickly escalated when protesters, who had sought to stage a rally outside the Moscow mayor’s office, defied police orders to disperse. On Tverskaya Street, where the office is located, law enforcement personnel were assaulted with pepper spray. Six officers were injured in the incident, according to media reports.

“The gas was sprayed from above, probably from one of the balconies of a nearby building,” a police source told TASS. The officers even had to wear gasmasks for a period of time, the Russian media reported.

In a separate incident, protesters also pelted officers with stones and sought to break through a police cordon on several occasions. The demonstrators blocked several streets in the city center near the mayor’s office, and vandalized some outdoor dining areas, according to TASS.

The police had to respond by forcibly dispersing the crowd. Almost 300 people were detained, police said in a statement. Some activist groups claimed, though, that the number of those arrested exceeded 500. Most of those detained were not Moscow residents, according to police.

In total, some 3,500 people took part in the action in defiance of a ban by the Moscow authorities. Earlier, Mayor Sergey Sobyanin warned Muscovites that the rally could be used for provocations, and advised citizens not to attend.

Saturday’s protest was called by an opposition activist, Aleksey Navalny, during another demonstration last weekend, which was sanctioned by the Moscow authorities. He was detained on Wednesday and sentenced to a 30-day administrative arrest for organizing an illegal event. Some other opposition activists were also detained either before or during this Saturday’s rally.

The ongoing series of protests in Moscow started two weeks ago after the Central Election Commission disqualified a dozen candidates wishing to run for the city council in September. The commission argued that they had forged voter signatures that they were required to submit as proof of public support. The disqualified candidates claimed the assessment process was flawed and called on the public to join the protests to make the authorities reconsider.

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