A Russian blogger who posted a tweet that said the children of police could be targeted in “snuff videos” has been sentenced to five years in jail, as three more Kremlin critics were charged with organising an illegal protest.

The tough sentence for blogger Vladislav Sinitsa came amid a flurry of legal moves in response to political protests over upcoming Moscow city council elections. More than a dozen people have been charged with rioting and attacking police officers in a sprawling investigation that has been dubbed the “Moscow case” and is seen as an attempt to dissuade further protests.

On Tuesday, a court sentenced Ivan Podkopaev, a 25-year-old technician, to three years in prison after he pleaded guilty to using pepper spray against police officers during a protest on 27 July. Also on Tuesday, investigators dropped charges against five of the 14 suspects in the case.

On Monday evening, three protest leaders were detained before the 8 September elections, which were already marred by the disqualification of independent candidates.

Ilya Azar, a protest leader and city council member, was taken by police from his home, leaving his young child alone in an unlocked apartment for several hours.

“They don’t care that a child who is not even two years old was left at home alone,” Azar, a municipal deputy who works for Russia’s Novaya Gazeta newspaper, said on Telegram.

“I cannot find the words to express my fury,” his wife wrote in a Facebook post, saying she came home to find the door open and her son “sound asleep.

Azar and two others, Lyubov Sobol and Nikolai Lyaskin, were released several hours later. They were charged with organising an unsanctioned rally and could face 30 days in jail.

Anger over the elections has led to the largest protests in Russia for years, with tens of thousands of people turning out to protest and police detaining more than 2,500 during a month of opposition rallies.

It has also led to a flurry of court cases against protesters, including several abortive attempts to take young children from their parents by depriving them of their parental rights. Several protest leaders have been held in jail for more than a month on consecutive charges of organising unsanctioned demonstrations.

Monday’s five-year sentence for Sinitsa was the first prison term handed down as a result of the recent unrest, and marked a new readiness of the government to crack down on online content.

When protests began in July, demonstrators used facial recognition apps to identify several of the riot police officers making the arrests.

According to police, Sinitsa, a 30-year-old blogger from the Moscow region, posted a tweet on 31 July under the pseudonym Maks Strelkov in response to a tweet asking what demonstrators would do with the facial-recognition data they gathered about police.

“They’ll look at their cute, happy family photos, study their geolocation, and then the child of the valiant defender of public order will not come home from school one day,” Sinitsa wrote, according to police. “Instead, a compact disc with a snuff video will arrive by post.” Snuff videos are homemade films of murder.

The prosecutor claimed that Sinitsa had called for “the kidnapping of the children specifically of law enforcement officers with subsequent acts of violence against them” and said he should be sentenced to six years in prison.

Pro-Kremlin activists largely welcomed the verdict. Vladimir Soloviev, a television host on the state-run Rossiya-1, said before Tuesday’s sentencing: “If Sinitsa gets six years, it will be a very clear signal to those who believe that the internet is a separate world and that no one can be found and punished. The time for jokes has ended.” He said he expected further cases to be brought for online threats.

“Sinitsa wrote a very stupid tweet about the need to take revenge against the children of police officers,” wrote Alexey Navalny, an opposition leader. “I get told 30 times a day that me and my children should be killed. But Sinitsa just got five years in prison for a stupid tweet. Our government is a cannibal and an idiot.”