"Bloody hell! A 15-day sentence for a one-man picket," Azar wrote on the popular messaging app Telegram following a court ruling.
Police detained the 35-year-old municipal lawmaker and journalist for the independent Novaya Gazeta newspaper on Tuesday.
He was arrested during a demonstration in support of activist Vladimir Vorontsov, who has worked to expose violations within Russia's law enforcement agencies.
Vorontsov, a former policeman, was arrested in early May on extortion charges. He was later accused of also illegally distributing pornography.
Vorontsov has denied the charges, saying police are seeking to punish him for his activism.
Police said Tuesday that Azar was being detained for violating a ban on rallies during Moscow's city-wide quarantine to slow the spread of coronavirus.
Azar's arrest sparked a backlash among his allies who said they planned to hold pickets in his support later Thursday.
"Azar had every right to hold this rally according to law, the constitution, and common sense," leading opposition politician Alexei Navalny wrote on Twitter.
Russia has recently cracked down hard on opposition demonstrations, and the single-person protest is the only kind allowed without prior permission from the authorities.
Azar last summer helped organise a series of anti-government rallies in Moscow demanding fair elections which drew tens of thousands of people.
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