Moscow, Russia - Thousands of people have flocked to rallies across Russia in support of opposition leader Alexei Navalny, in an event that coincided with the birthday of President Vladimir Putin.

In the Russian capital, Moscow, two protesters were briefly held by police but were later released. Across the country, police detained at least 139 people in 25 cities, according to OVD-info, a group monitoring political repression.

In Moscow, thousands gathered at Pushkinskaya Square under a heavy police presence, chanting: "Putin will leave! We will not!" and "Happy birthday!" The crowd attempted to march towards Manezhnaya Square near the Kremlin but was prevented by police.

"I wanted to come out on Putin's birthday to protest. He has done already a lot of harm. I'm against him and for free elections," Inna Kavatsun, a 55-year-old retired real estate agent, told Al Jazeera.

Ivan, an 18-year-old university student who did not provide his last name, noted that protesters wanted "to express our disagreement with what is going on in this country".

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"Alexei Navalny, after he announced he wants to run for president, started getting constantly arrested and harassed. We have come out to express our rejection of that," he said.

"I support Navalny because he offers some alternative. Throughout my life, there haven't been any fair elections. It has always been Putin. We never saw him join debates or present his platform. How can we support him? ... If people don't walk out, nothing [will] change."

Navalny had called on his supporters to come out on Saturday to support his campaign and demand that he be allowed to run in the 2018 presidential election, after he was arrested on September 29.

A local court accused him of violating the law on organising a number of other demonstrations, and ordered his detention for 20 days. Protests were subsequently arranged in 80 cities across the country; some were not sanctioned by local authorities, including in Moscow and St Petersburg, Putin's city of birth.

According to Russian law, organisers of demonstrations and rallies have to obtain permission from local authorities. Coordinators for Navalny's campaign in Krasnodar, Tver and Smolensk have been arrested for organising "unsanctioned" events, while police have also searched Navalny's campaign headquarters in Moscow, St Petersburg and Kaliningrad.

On Friday, opposition party Yabloko announced that it did not support the protests. In a statement published on the party's website, vice-chair Nikolay Rybakov said: "Such 'events' are clear provocations, aiming at suspect PR promotion of certain personalities. The main point is that youth disturbances don't have a political meaning and don't change anything."

In March and June, tens of thousands of people across Russia responded to Navalny's call for protest against government corruption; more than 1,500 people were detained each time.

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Navalny announced his presidential campaign last December, but in June, the head of the Central Electoral Commission said he could not run because of a previous criminal conviction for fraud - charges that he says were fabricated for political reasons.

In 2016, the European Court for Human Rights ruled in his favour, and a higher court in Russia sent back his case for retrial. Last February, he was given a five-year suspended sentence.

Vitaly Serukanov from Navalny's Moscow campaign team told Al Jazeera that they still consider him to be a viable candidate, and he will continue campaigning.

In mid-September, Navalny started a tour around the country to rally supporters, visiting six cities before his arrest.