Russian opposition leader Alexei Navalny announced on Thursday that he had been released from custody after completing a 30-day sentence for organizing unauthorized protests ahead of President Vladimir Putin's fourth inauguration last month.

"I'm with you again after a 30-day business trip. I'm so happy to be free," he wrote on Twitter.

He walked free from a Moscow prison just as Russian capital was preparing open the FIFA World Cup later on Thursday.

Read more: Opinion: Navalny raised the stakes too high

The politician had called on supporters to take to the streets and challenge Putin's election last month under the slogan "Not our tsar." Navalny was barred from running in the election by the Russian constitutional court due to an embezzlement conviction from 2014.

The Russian court's decision contradicted an earlier ruling from the European Court of Human Rights, which found the conviction to have been "arbitrary."

Putin-critic Alexei Navalny photographed leaving prison in Moscow

Anti-Putin protests met with crackdown

Last month's anti-Putin demonstrations saw around 1,600 protesters detained across 27 Russian cities, according to OVD-Info, an independent monitor that tracks political repression in Russia. Footage showed police beating protesters with truncheons and dragging them along the ground.

Navalny was arrested by police and carried away by his arms and legs moments after appearing in Moscow's packed Pushkin Square.

The opposition leader and his lawyers maintain that the arrest and trial were politically motivated and that he had had his rights violated. The European Union also condemned the "police brutality and mass arrests" following the demonstrations.

Read more: Putin 4.0: What to expect from the Russian president's next term

Navalny has faced a string of charges since becoming Russia's leading opposition figure and a vocal anti-Putin campaigner. Just last year, he served three separate jail sentences for breaking rules on organizing demonstrations.

He also had to travel to Spain for surgery after one of several street attacks nearly blinded him in one eye.

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dm/sms (AP, AFP)