Moscow (CNN) Thousands of people marched through the streets of towns and cities across Russia Monday to protest against corruption.

Opposition leader Alexey Navalny, who called for nationwide demonstrations to mark Russia Day, was detained outside his Moscow office after police declared that his plan to direct protesters down one of the capital's main streets was illegal.

Navalny's YouTube channel, which had been broadcasting live to more than 50,000 people as the protests got underway, lost light and sound in the studio.

The 41-year-old has been mobilizing support on social networks, and hopes the rallies will rattle the Kremlin, as those held earlier this year did.

Navalny posted his first YouTube video, a step-by-step instruction guide showing how to build an "agitation cube," a box-like tent structure with his image emblazoned on the side, in July 2013.

The clip marked the start of the Russian dissident's campaign to be elected Moscow mayor, and the humble beginning of his YouTube revolution.

Navalny Live: 'People are sick of not being heard'

Navalny was arrested in Moscow during anti-corruption protests in March.

Oksana Baulina, who now runs Navalny Live, the live-streaming companion to the original Navalny YouTube channel, worked on that mayoral bid.

Wearing an old campaign t-shirt which says "Only Navalny, only hard work" in big, black letters, Baulina says the events of 2013 give her hope.

"Experts or even supporters didn't believe that Navalny would be allowed to take part in the mayoral elections," she says, let alone that he'd come away with 27% of the vote, narrowly falling short of a runoff.

Navalny has now set his sights on Russia's 2018 presidential election, despite being found guilty of embezzlement and given a 5-year suspended sentence in February. Russian laws prohibit convicted criminals from running for office. Navalny says the charges against him are politically motivated.

The Navalny Live team, based just outside Moscow city center, is also undeterred. "Navalny 2018" stickers are affixed to every laptop, and boxes of campaign signs are stacked in the corridor. At one point, Navalny himself wanders into the studio to check on things.

Leonid Volkov, Navalny's presidential campaign manager, is anchoring a lively discussion program about internet freedom. Throughout the broadcast, Baulina types furiously. This is not just about sending a message to the viewers, she explains. During every show they coordinate comments on YouTube and conversations on Twitter using hashtags.

People are "sick and tired" of not being heard, she says. "They need this."

The numbers are impressive: more than 300,000 people now subscribe to Navalny Live, which produces up to three broadcasts a day each week; the original Navalny YouTube channel has passed the 1 million mark.

Navalny's most-watched video is a 50-minute graphics-heavy investigation into alleged corruption by Prime Minister Dmitry Medvedev, which has notched up well beyond 20 million views. Medvedev has denied the allegations in the video.

At Navalny Live, the mission that day was to stream the protests as they happened, since state-run media was not covering them at all. Starting early, with the protests in Russia's far east, Baulina's team worked their way across the country's time zones. By the time the live stream reached Moscow, she says, it had peaked at 140,000 simultaneous viewers.

And then there was a knock at the door.

Under the pretext of failing to comply, first with a fire alert, and then with a bomb threat, Baulina and several of her colleagues were arrested and detained for seven days.

It wasn't the most comfortable experience, she says, but it only served to strengthen her resolve.

"Actually, after you go through the first arrest, you realize it's not as difficult as you thought it would be." The point, she says with a smile, is "to build a beautiful Russia of the future."

'I spit on you, Navalny'

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There are signs, though, that the ground may be shifting beneath this opposition movement.

In a rare break from the Kremlin's long-held policy of appearing to ignore Navalny, long-time Vladimir Putin ally and billionaire oligarch Alisher Usmanov -- named by Navalny as an accomplice in Medvedev's alleged web of corruption -- decided to take him on at his own game.

In two rambling video messages on his own newly-created YouTube channel, Usmanov called Navalny a liar and promised to sue him for defamation -- something he has now successfully done.

His payoff lines -- versions of "I spit on you, Navalny" -- have become something of an internet meme in Russia.

Baulina says she thinks this is a good thing, though she worries it might simply be a ploy by Usmanov to take the heat off Medvedev: "It somehow shows that it's not us who follow their agenda, but they who follow ours."

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And then there was the highly produced, not to mention highly controversial, video entitled "Hitler 1945/Navalny 2018," which racked up two million views on YouTube before it was deleted. Navalny believes the video was the work of the Russian government. Russia's independent TV channel Dozhd came to a similar conclusion, citing four sources close to the Kremlin who say the Kremlin would try to undermine Navalny. Kremlin spokesman Dmitry Peskov denied any involvement in the video.

Navalny's live video operation is not a high budget affair. Brightly-colored paper screens roll down to form the backdrop, and there are four dedicated employees.

During CNN's visit to the station, a woman in her 60s arrived nervously clutching a carefully-wrapped gray plastic package. Inside was cash, she said, to donate to Navalny's campaign. Staff were forced to explain that they can only take electronic donations.

"But I don't like using a computer and sharing my thoughts with anyone," the would-be donor pleaded. "I learned how to open Navalny Live on YouTube -- that's enough for me."

In a country where opposition is so tightly controlled, YouTube is an increasingly powerful tool. Whether it will be enough for Navalny is still highly debatable.

This story has been updated to reflect developments on the ground in Russia.