For years, the Kremlin has publicly dismissed Alexey Navalny as an irrelevant blogger with little popular support. But the geographical spread of mass protests over two weekends in support of Navalny, with thousands turning out in dozens of cities across Russia, underlines how much of a challenge he and his anti-corruption message have now become for the Kremlin.
A recent exposé on an extravagant secret palace on Russia's Black Sea coast, which an investigation by Navalny's Anti-Corruption Foundation (FBK) said had been built for President Vladimir Putin, has more than 105 million views on YouTube, and helped fuel public outrage at what the FBK describes as official excess. CNN is not independently able to verify the FBK's claims. Putin's spokesperson Dmitry Peskov has denied the Russian leader was linked to the estate.
But it's the harsh treatment of Navalny, 44, and his bravery in the face of it, which really seems to have struck a chord.
Many Russians are outraged by Navalny's horrific nerve agent poisoning in Siberia last year, which Navalny says was ordered by Putin and carried out by agents of Russia's security service, the FSB. The Kremlin has repeatedly denied the allegations.
The fact that Navalny flew back to Russia from Germany after he recovered, only to face immediate detention by the Russian authorities -- and possibly a long prison term -- further galvanized sympathy, united Russia's often splintered opposition and cemented his status as a serious threat to the Kremlin.
Just how serious is unclear.
The latest nationwide protest Sunday drew thousands of Navalny supporters on to the streets across Russia. More than 5,000 people were detained by police determined to thwart the unsanctioned demonstrations, according to OVD-Info, an independent site that monitors arrests.
Putin has faced mass street protests in the past, notably between 2011 and 2012, when tens of thousands protested against parliamentary elections which critics say were flawed, and against Putin's decision to return to the presidency after a brief spell as prime minister, circumventing a constitutional bar of more than two consecutive presidential terms. Calls for Putin to step down were ignored, and the Russian government last year pushed through constitutional amendments -- ratified by a referendum -- that potentially allow Putin to stay in office until 2036.
In the past, Kremlin opponents have been gunned down, poisoned or discredited in a bid to silence them. The Kremlin has always denied involvement.
Boris Nemtsov, a leading Russian opposition figure, was shot near the Kremlin in 2015 to international condemnation. He was then considered the most visible leader of the Russian opposition. Putin quickly condemned the killing, the Kremlin said at the time. Five Chechen men were found guilty by a Russian military court and were handed prison sentences for his shooting in 2017.
In 2006, Russian investigative journalist Anna Politkovskaya was gunned down in her Moscow apartment building. Five men were convicted over her murder but her former colleagues have said there is little hope that whoever who ordered the killing will be held to account. The Kremlin denied any connection to the killing.
Russia's former richest oligarch, Mikhail Khodorkovsky, served more than a decade in jail. Convicted of tax evasion and fraud -- charges he has argued were politically motivated -- Khodorkovsky saw the dismantling of his oil company after he backed opposition groups and spoke out about official corruption.
Former Russian agent turned Kremlin critic Alexander Litvinenko died in Britain in 2006 after being poisoned with a rare radioactive isotope, polonium-210. A UK inquiry concluded in 2016 that Putin probably approved the operation by two Russian agents to kill Litvinenko. Russia's Foreign Ministry at the time dismissed the UK inquiry as politically motivated.
Navalny is only the latest in a line of challengers to Putin's power.
An early test will come on Tuesday, when a Moscow court is scheduled to decide if Navalny broke the terms of a suspended sentence for an embezzlement conviction, dismissed as politically motivated by critics.
If the court decides against Navalny, he may be ordered to serve the 3.5-year term behind bars.
Much will also depend on actions of the international community in general, and US President Joe Biden's new administration in particular.
There have been strong words of condemnation from the Biden administration and from Secretary of State Antony Blinken, even a direct telephone call between the US and Russian Presidents in which Navalny's treatment was raised.
But so far there's been no action.
After years of relentless criticism in the media and in Washington of how former US President Donald Trump was soft on Putin, all eyes are watching whether Biden will get tough.