Russian opposition leader Alexei Navalny has been given a 20-day prison sentence for repeatedly violating a ban on organising public meetings.
His supporters say he was detained to stop him taking part in presidential elections in March when Vladimir Putin is expected to seek a fourth term.
It is the third time this year that Mr Navalny has been jailed, according to BBC report.
Russia's electoral authorities say he cannot stand in the March vote because of a separate suspended sentence.
That conviction for embezzlement was condemned last month by the Council of Europe's committee of ministers as "arbitrary and unfair".
His latest sentence came after he tried to organise a rally on Friday in Nizhny Novgorod, about 400km (250 miles) east of Moscow. The authorities said he did not have permission to organise such events.
A close associate, Leonid Volkov, was also jailed for 20 days and threatened to go on hunger strike.
Alexei Navalny, one of Russia's most prominent anti-corruption campaigners, led mass street protests against President Vladimir Putin in the winter of 2011-12.
In recent months he has travelled across Russia in a bid to bolster his makeshift election campaign. He had been due to attend a major rally in Mr Putin's home city of St Petersburg on Saturday.
Speaking in court on Monday he described his latest punishment as "a gift to Putin for his birthday" and urged his supporters to protest in St Petersburg and elsewhere.
"Do not give up, keep resisting," he said. "We do not want to see in power these impudent, hypocritical, deceitful, stupid loafers who fancy themselves gods. They are not gods. We are the masters of our country!"
His lawyers say they will appeal against the sentence.
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