MOSCOW – Aleksei A. Navalny returned to his home country on Sunday, five months after a near-fatal nerve agent attack and was arrested at the border. This is a sign of the fearlessness of Russia’s most prominent opposition leader and the concern of President Vladimir V. Putin.

In hours of live streaming drama that took place in Berlin, in the air and at two Moscow airports, Mr Navalny fell headlong into near-safe custody after deciding to leave the relative security of Germany, where he fell from the last Summer had recovered from poisoning.

Hundreds of people brave the bitter cold outside Moscow’s Vnukovo Airport to greet Mr Navalny, but the cheap Russian airline he was flying was diverted to another Moscow airport just before landing. There, Mr. Navalny was confronted with uniformed police officers in black masks during passport control.

He hugged his wife Yulia Navalnaya before being led away.

“I’m not afraid,” Navalny told reporters shortly before his arrest, standing in front of a neon sign at the airport depicting the Kremlin. “I know that I am right and that all criminal proceedings against me are fabricated.”

The arrest of Mr Navalny had been expected, but the day presented some of the most dramatic images of the past few years, underscoring both Russia’s growing domestic dissatisfaction and the Kremlin’s unrest over it.

Countless riot police in camouflage uniforms and shiny black helmets swarmed the arrival halls of Vnukovo and detained dozens. Other officials, some in plain clothes, came across some of Mr. Navalny’s finest employees while they were dining at an airport cafe and leading them away.

Russia’s independent media offered uninterrupted live coverage, which was freely available on Russia’s mostly uncensored Internet, from the moment German police officers escorted Mr. Navalny onto the asphalt in Berlin. Dozhd, an online television station, reported that its live feed was viewed six million times on Sunday night.

Always aware of the social media look at home, Mr Navalny responded in Russian to questions he was asked in English when he boarded the plane in Berlin. Shortly before the start, he published a video on Instagram in which his wife delivered a line from a popular Russian crime thriller: “Bring us vodka, boy. We’re going home. “

His style – tough, populist and humorous at the same time – contributed to the 44-year-old Navalny becoming Russia’s most famous opposition leader. An online audience of millions watches his YouTube videos showing corruption rife among the ruling elite.

But his followers aren’t the only ones watching.

In August, Mr. Navalny was poisoned in Siberia by a military grade nerve agent. He and Western officials said it was an assassination attempt by the Russian state.

In December, after an investigation by the Bellingcat research group, Mr Navalny pretended to be a Russian officer and called a security agent who was part of the unit that tried to kill him and extracted what sounded like a confession.

However, last Wednesday, Mr Navalny said he was coming home despite the threat of arrest. “Russia is my country,” he said. “Moscow is my city. And I miss her. “

The question now is whether Mr Navalny will only be detained for a few days or weeks – as has happened to him repeatedly in recent years – or for much longer.

Shortly after his arrest on Sunday evening, the Russian State Prison Service announced that Mr. Navalny would remain behind bars pending a trial for violating the terms of a suspended sentence he originally received in 2014. The sentence arose from a financial crime case brought against him and his brother, which the European Court of Human Rights later found unjustified.

According to the prison service, Mr Navalny did not report twice a month during his recovery in Germany last year, as requested by the court. In the days leading up to his return home, the service warned that he would be arrested for these reasons.

Mr Navalny’s fate may depend in part on the intensity of the backlash to his arrest at home and abroad. In Russia, his supporters called for protests in the coming days and found that his lawyer had not been given access to the opposition leader.

“Aleksei Navalny was kidnapped, he is in danger,” a senior adviser to Mr. Navalny, Leonid Volkov, posted on the telegram a few hours after his arrest. “He’s in the hands of people who have tried to kill him.”

In the United States, Jake Sullivan, national security adviser-designate to President-elect Joseph R. Biden Jr., posted a Twitter request for the immediate release of Mr. Navalny: “The Kremlin’s attacks on Mr. Navalny are not just a violation of human rights, but an affront to the Russian people who want their voices to be heard. “

Outgoing Secretary of State Mike Pompeo also condemned the arrest. “Aleksei Navalny is not the problem,” he said in a statement. “We demand his immediate and unconditional release.”

Mr Putin, who has ruled for 21 years, retains tight control over television waves, domestic politics and an extensive security apparatus. But its popularity with the Russian public has waned in recent years amid stagnant incomes and widespread aversion to official corruption.

Mr Navalny has taken advantage of the discontent, built a nationwide network of local offices, and used social media to highlight the hidden wealth of the elite and the struggles of regular Russians.

Vladimir Murzin, a 50-year-old legal advisor, was among the supporters who wanted to greet him at Vnukovo Airport on Sunday. Mr Murzin said he and several others had come from Tambov – a 300 mile drive – to be there. The poisoning of the opposition leader only intensified his “years of anger over the injustice of what is happening in our country under the Putin regime”.

“This is a man the masses will follow,” said Mr Murzin of Mr Navalny. “Any citizen who does not agree with the current regime needs mutual support.”

But Mr Navalny’s flight on the Russian state airline Pobeda – which means “victory” – never made it to Vnukovo.

As the Boeing 737 approached Moscow, air traffic controllers radioed the flight’s pilots and said the plane could not land because of a blocked runway. The flight – and three others – was diverted to another Moscow airport, Sheremetyevo.

An official statement later blamed a stuck snowplow for the diversions. But it seemed like a transparent ploy by the Russian authorities to defuse the protests of the Navalny supporters gathered in Vnukovo.

“This shows once again what is happening in Russia,” said Navalny after his flight was rerouted and apologized to his fellow passengers for the inconvenience. “The rulers are not only disgusting thieves, but also totally pathetic people who spend their time with utter nonsense.”

The scale of the operation to cope with the opposition leader’s return contradicted Putin’s insistence that Mr Navalny is of minor importance. In December, Putin denied that the state had anything to do with the poisoning of Mr. Navalny, saying, “Who needs him?”

Mr Navalny – who was banned from running for the presidency in 2018 – has warned Russians to use elections to lose Putin’s power by voting for the best-positioned opposition candidate, even though the votes are not free and fair. The next test of this strategy will take place in September, when national parliamentary elections are scheduled.

Last year, Putin gave himself the opportunity to rule until 2036 by making constitutional changes that allowed him to run for two more terms. At the Moscow airports where the drama took place on Sunday, some of his opponents admitted that achieving political change in their country seemed increasingly to be a long, dangerous and potentially bloody road.

“It will be necessary to sacrifice many lives,” said Svetlana A. Utkina, a 52-year-old Russian teacher and supporter of Navalny, in an interview in Sheremetyevo shortly after the opposition leader was arrested there.

“I’m a pessimist and an idealist,” she said. “Because if you keep squeezing people for a long time, people’s fear will eventually be suppressed.”

Mr. Navalny’s wife was not arrested, and the arrival hall burst into chants of “Yu-li-a!” when she got out of customs without her husband.

A crush of journalists followed her into the Moscow night outside the airport. Shortly before getting into a car, she said, according to video footage from the scene, “The most important thing Aleksei said today is that he is not afraid. I am also not afraid and I urge you all not to be afraid. “

Oleg Matsnev and Sophia Kishkovsky contributed to the research.