“DON’T go,” I said out loud to the radio on hearing the news that the Russian anti-corruption campaigner Alexei Navalny was leaving Germany — where he had spent weeks recovering from his poisoning with Novichok — to return to Moscow, where he faced arrest, imprisonment, and possibly something worse.

But Mr Navalny is made of sterner stuff, which is what has made him the most prominent opponent of President Putin. The 44-year-old blogger, who has millions of followers on Russian social media, has declared that President Putin’s political party is full of “crooks and thieves”, and has accused the President of “sucking the blood out of Russia”. For nearly a decade, he has promoted illegal non-violent protests calling for political reform. On landing back in Moscow, he was immediately arrested at passport control, pausing only to kiss his wife as he was led away by police.

Opposition leaders in Russia usually end up in prison, in exile, or dead. But Mr Navalny is undeterred. While in recovery, he rang one of the Russian security service’s hitmen and tricked him into revealing that the Putin regime had planted nerve poison in his underpants. Then, within hours of his return to Moscow, he published a two-hour film, Putin’s Palace, which exposed, in hugely embarrassing detail, the building of a luxurious $1.3-billion personal residence by the Black Sea, complete with casino, pole-dancing room, and underground ice-hockey rink. The film has had 90 million internet views, forcing President Putin into a public denial that the palace is his.

Mr Navalny’s arrest has resulted in large-scale protests in 100 Russian cities — from St Petersburg on the Baltic to Vladivostok on the Pacific. Protesters even braved temperatures of -60ºF in Yakutsk, the world’s coldest city. Everywhere, the shouts have been: “Putin is a thief! Freedom to Navalny!”

The Putin regime purports to be untroubled. Its officials claim that they perceive no real threat to their two-decade-long grip of power. After all, President Putin survived big street protests when he returned to the presidency in 2012. And he has weathered the international condemnation and economic sanctions that followed his annexation of Crimea in 2014.

But the current nationwide wave of protest is being brazenly applauded by ordinary folk on the tram as they pass. Motorists are honking their car horns in approval. Those on the streets are largely Russians in their twenties who get their news from social media, not state television. The opposition is strongest among professionals and the middle class. In Moscow, about one third of the capital’s residents are said to oppose the government.

Kremlin officials dismiss Mr Navalny as a figure of little public significance, and insist that opinion polls show that the President remains popular. Yet if he is truly no threat, why was Mr Navalny prevented from standing against President Putin at the last election? And why are ranks of baton-wielding riot police responding with increasingly heavy-handed tactics and arresting thousands of Navalny supporters?

The only logical conclusion is that President Putin has been rattled by the emergence of a younger opposition figurehead, who is not only an articulate speaker and a clever campaigner, but also a man of extraordinary courage.