What they left behind showed the brute force of their adversary: a single black shoe, a snapped pair of glasses, a shiny hair clip.

Thousands of people, many in their teens and twenties, poured into the center of Moscow on Monday to protest against the corruption they say eats away at their livelihoods and future. The rally was organized by opposition leader Aleksei Navalny, a charming 41-year-old lawyer who has designs on the presidency.

One by one, hundreds were picked off by riot police, who dragged and pulled them by their arms and legs, some by their hair. I watched as police sucker punched gangly adolescents and hit them with batons before tossing them into paddy wagons. The crowds were resilient, hissing and booing at police officers as their fellow protesters were manhandled. Several youths beside me held up a handwritten placard: “Only revolution will defeat corruption.” Others chanted, “Make Russia free!” The demonstrators waved smartphones and GoPros in the air, trying to capture the injustices.

The protest wasn’t meant to happen this way. Mr. Navalny, in a last-minute decision, changed the location of the rally from the north of Moscow to the city center, saying authorities had intervened and pressured the companies providing his stage and sound equipment to abandon him. But while he received the necessary government permission for his original protest, he did not secure it for the second. In Russia, Soviet-style bureaucracy means that if you don’t get a green light from authorities before holding a protest, you risk arrest.

Mr. Navalny himself barely made it out of his house on Monday morning: He was detained, whisked off to court and sentenced to 30 days in jail for organizing an unsanctioned rally.

Does this mean Mr. Navalny is irresponsible? In the eyes of his devotees, it has made him a hero. He was cheered and celebrated on Monday. “It’s absurd that they try to make us scared of walking and existing,” said 20-year-old Valery, as police pushed deeper into the mass of people, pressing bodies against metal barriers. Taking over the heart of Moscow emboldened some protesters. At the top of Moscow’s main drag, Tverskaya, riot police tried to divide the bulging crowds into smaller groups. “You’re not real men!” sneered a teenage girl, her cheeks flushed with anger. A young man by the name of Stepan tried to argue with a policeman. “Come on, admit it. You’re not any happier with your life than the rest of us.” The policeman lowered his helmet’s visor, and told Stepan to move further down the road. When Stepan refused, the policeman shoved him.

When antigovernment protests, then the largest of their kind to oppose President Vladimir Putin’s rule, erupted in the winter of 2011, we thought it was the beginning of the end. I remember Mr. Navalny standing on a stage in Bolotnaya Square near the Kremlin, telling the crowd that Mr. Putin’s party was made of “crooks and thieves.” Thousands turned out, and hundreds were arrested. But then, just as the movement seemed to be gaining momentum, Moscow literally froze over, Russians became distracted by New Year celebrations and the protests petered out. The authorities seemed to listen, though, and dozens of Moscow parks were later renovated to appeal to hipsters, complete with dedicated spaces to hold free and open discussions.

But most of today’s protesters were children then. The thousands who descended on Moscow today are young, some of them very young, and have known virtually no other power but that of Mr. Putin, who has ruled for 17 years. They are fed up. Their leader Mr. Navalny summed up their frustration last week in his blog, writing, “I want to live in a modern democratic state and I want our taxes to be converted into roads, schools and hospitals, not into yachts, palaces and vineyards.”

June 12 is also Russia Day, a national holiday, and Moscow had made preparations for family activities across the city. City authorities spent weeks, possibly months, in preparation for the events. And in a split second, in a late-night address on his YouTube channel, Mr. Navalny managed to change it all. Among the throngs of people waving Russian flags on Monday, it was difficult to tell who was protesting, and who was trying to enjoy the day off. Perhaps that was part of Mr. Navalny’s plan.

Mr. Navalny has spent the past several years being harassed endlessly by the Kremlin. His campaign offices have been habitually bombarded and shuttered. His eyesight has been irreparably damaged by unknown assailants. And he has spent months under house arrest. For such a man to command that kind of power — enough to shut down the center of Moscow at his word — is something the Kremlin would be hard-pressed to ignore.