Aleksei A. Navalny at a court in Moscow on Monday. He told reporters that he was “amazed” by the number of cities and by how many people had taken part in demonstrations.
MOSCOW — A day after the largest antigovernment protests in more than five years, a Moscow court on Monday slapped the opposition leader behind the outburst, Aleksei A. Navalny, with a 15-day prison sentence for resisting arrest.
At the urging of Mr. Navalny, tens of thousands of Russians — many of them in their teens and 20s — poured into the streets in scores of cities across the country on Sunday to protest endemic corruption among the governing elite, despite a blanket ban against unsanctioned rallies of any size.
The police responded by beating protesters and arresting more than 1,000 in Moscow alone, though by Monday many had been released.
As Mr. Navalny was led into the courtroom for a hearing that lasted much of the day, he told reporters that he was “amazed by the number of cities that took part in this and by how many people came out.” After the judge ordered him jailed, Mr. Navalny was whisked away without being allowed a chance to comment further.
In another potentially destabilizing development, Russian truck drivers across the country began organizing their largest concerted protest since December 2015, against a new toll system on national highways. Clusters of transport vehicles were parked on the shoulders of highways near major cities in what looked like preparations for roadblocks.
Mr. Navalny, 40, a charismatic opposition figure, has long been caught up in multiple court cases that he calls politically motivated. He has vowed to run against President Vladimir V. Putin for the presidency in 2018, even though a previous, trumped-up conviction makes that technically illegal.
The government has usually avoided jailing him, however, apparently fearful of turning him into an even bigger political martyr. The court on Monday also fined him about $350 for organizing an illegal demonstration.
The protests provoked surprise and wonder among analysts and activists, many of whom had written off the public as politically apathetic. The high number of young people in the crowds struck many as particularly impressive.
“Right now it looks like a major new phenomenon in Russia that there are young people who are active and have agendas,” Maxim Trudolyubov, a columnist, said. “One important factor is that these people all grew up under Putin. They don’t remember any other leader.”
If Mr. Navalny has breathed new life into the opposition movement by successfully getting thousands of protesters onto the streets in what he said was 99 cities, there was no immediate threat to Mr. Putin’s standing. The Russian president’s approval rating reached around 86 percent in the wake of the 2014 annexation of Crimea and has barely budged since.
Mr. Putin, as the “good czar,” enjoys a status that the rest of the government falls short of. Mr. Navalny, however, in calling for people to come out to protest high-level corruption, rather than the president, appears to have tapped into a popular sentiment.
It remained to be seen if the rallies would spread or die out. After the last protests demanding stronger democracy, which took place in Bolotnaya Square in Moscow beginning in December 2011, the government threw the book at a number of demonstrators. Jail sentences of at least three years effectively chilled the enthusiasm of the predominantly middle-aged participants.
The aftermath of the rallies will show whether there is a political thaw or not, wrote Oleg Kashin, another columnist. If there is a new Bolotnaya case, he said, then it is not a thaw; otherwise, it is, even if no substantial policy changes follow.
There were no independent numbers available for the turnout for the rallies. The liberal radio station Echo of Moscow estimated that more than 60,000 people had participated.
Analysts cited a number of factors for the surprising strength of the protest: pent-up anger over government suppression of peaceful demonstrations; some rankling over the dreary economy; and the weather, because Sunday was sunny and relatively warm for March.
But overwhelmingly, they cited the youthfulness of the crowds, with many presumably participating in their first political protests.
“A milestone event occurred in Irkutsk today — a new politically active class was born,” Aleksei Petrov, a longtime election monitor in the Siberian city, wrote on Facebook. “Almost a thousand young people of student age have taken to the streets.”
The young are less likely to watch state television, which endlessly exalts Mr. Putin and government policy. Indeed, all of the state-run channels ignored the rallies. Instead, young Russians tend to turn to independent voices who broadcast their own shows on YouTube.
The immediate inspiration for Sunday’s protest was a nearly 50-minute video compiled by Mr. Navalny and his colleagues and released on March 2. It detailed what they called a web of dubious charitable organizations that funneled bribes from prominent oligarchs to Prime Minister Dmitri A. Medvedev, allowing him to maintain a series of luxurious estates, vineyards and yachts in Russia and abroad. It has been viewed more than 13 million times.
“I am glad and happy that a new generation grew up in the country that will not accept such attitudes from the government and wants to feel that they are citizens,” Mr. Navalny told reporters in court. “This was news to many yesterday, including me.”
Victor Zadorozhnyi, 26, a real estate broker who was attending his first political rally in Moscow, said: “Have you noticed that only young people are around? Why so?”
He predicted, accurately, that none of the events would appear on Channel One, a main government mouthpiece.
“Younger people have smarter eyes,” he said. “Elder people, they have rigid brains; watching Channel One is a routine for them — they switch it on to see what is Channel One telling us. Everything’s good, Mashka the cat had kittens yesterday.”
For its part, the Kremlin denounced what it called illegal rallies, though the organizers maintained they had permits for 21 of the protests.
“The Kremlin respects the civic stance of the people and their right to voice their position,” Dmitri S. Peskov, the spokesman for Mr. Putin, told reporters. “We can’t express the same respect for those who consciously misinformed people.”
Mr. Peskov accused the opposition of jeopardizing and exploiting “children” by promising them some manner of reward for attending. It was not clear what he was referring to, though Mr. Navalny had offered to help anyone arrested to seek redress and compensation through the European Court of Human Rights.
Mr. Peskov defended the actions of the security forces, who often grappled with demonstrators and seemed to cart off some who had not been participating in the protests. “Law enforcement was acting in a totally correct, highly professional and lawful way,” he said.
He denied that the Kremlin had ordered a news blackout on state-run channels, even though none of them reported on the rallies. “We don’t shape the editorial policies of TV channels,” Mr. Peskov said.
The spokesman said the Kremlin was aware that the protests had not been concentrated in the main cities alone, another phenomenon not seen for years. “The Kremlin is quite sober when analyzing the scale of yesterday’s protests,” he said, adding that it would neither “diminish, nor exaggerate the numbers.”
The fate of all the arrested protesters in Moscow and other cities was not clear. The police put the figure at 500 arrests in Moscow alone, but an independent organization that tracks such detentions put the number at closer to 1,000. Most were released overnight.