Russia has opened a criminal investigation into the July 27 protest in Moscow where police detained almost 1,400 people, the Investigative Committee said on July 30.
The potential charges include organizing or participating in mass disorder, which could carry a sentence of up to 15 years in prison, the committee said in a statement.
The detentions were made as several thousand Russians took part in an unauthorized rally to protest officials who didn't register several independent and opposition candidates on the ballot for a September 8 vote to the Moscow municipal legislature.
The 45 members of the Moscow City Duma hold powerful posts -- retaining the ability to propose legislation as well as inspect how the city’s $43 billion budget is spent.
The police crackdown was one of the harshest in recent years against an opposition that has grown more defiant while denouncing Russian President Vladimir Putin’s hold on power.
Police in Moscow have been condemned by rights groups and many Western governments and organizations for a "disproportionate" use of force in breaking up the demonstration.
Moscow Mayor Sergei Sobyanin, in his first comments on the disorder, said on July 30 that the protesters "forced police to use force that in this situation was completely appropriate."
The committee said a criminal probe was opened based on three articles of Russia's Criminal Code -- on organizing mass unrest, which carries a sentence of up to 15 years in prison; on participating in such unrest, which can be punished with up to eight years in prison; and on calling for unrest, which carries up to two years in prison.
"The investigation has established that ahead of an unsanctioned rally, a group of people repeatedly posted Internet messages calling for people to take part in it, knowing full well that these actions could provoke mass unrest," the committee said.
Separately, the committee said it is probing violence against police, members of the Russian National Guard, and other officials.
No individuals were named as suspects so far.
More than 40 people have so far been handed jail sentences ranging from three to 30 days for attending the rally. The independent political watchdog OVD-Info said on its website the number is likely to rise as court hearings continue.
On July 30, opposition activist Dmitry Gudkov, one of those denied registration as a candidate, was sentenced to 30 days for what the court called “a "repeat violation of the established procedure for organizing and holding an assembly, rally, demonstration, procession, or picket."
Several other candidates rejected by the Moscow City Elections Commission have also been handed sentences after being arrested at the rally.
Moscow election officials said earlier in July that they had barred some opposition candidates from the September 8 Moscow City Duma election because of insufficient signatures on nominating petitions.
The decision sparked several days of demonstrations this month, including one on July 20 in Moscow that drew an estimated crowd of 20,000.
Despite the July 27 crackdown on the unauthorized rally, activists say they are undeterred and are looking to hold another demonstration on August 3, though talks with Moscow officials broke down on July 30.
In a sign of how tense the situation remains, the leader of Russia's Libertarian Party was detained by police after the talks with municipal authorities over the possible site of another rally.
Mikhail Svetov was arrested after leaving city offices, saying the location of the demonstration is the main issue.
“The municipality said that any other sites besides Sakharov Avenue are not being considered,” Svetov, whose party had filed a notice for a rally at Lubyanka Square in front of the headquarters of the Federal Security Service (FSB), said in a tweet.
“They also refused to consider other options for public actions, such as processions. Straight and tough ultimatum,” he added.
Aleksei Navalny, Russia's most prominent opposition activist who is currently serving a 30-day jail sentence for calling the latest protest, was taken to the hospital late on July 28 with severe swelling of the face and a rash, sparking fears he had been the victim of a poisoning attempt, though hospital officials characterized the illness as "a generalized allergic reaction."
Navalny, who is appealing his sentence, has said he shares suspicions that he may have been poisoned in prison and his lawyer, Olga Mikhailova, said on July 29 that she was asking for the court to terminate the case “due to the lack of evidence or to terminate his administrative arrest due to his poor health condition."
“He was really poisoned by some unknown chemical substance," she said. "But what the substance was has not been established."
The Kremlin critic also posted a picture of himself on social media with a bloated face and one eye shut that he said he couldn’t open.