Suddenly the Kremlin has a problem. Vladimir Putin's return to the top job after four years as Prime Minister was meant to go as smoothly as the 2008 mirror-image transition, when he stepped aside for Dmitry Medvedev, while remaining effectively in charge. But this time, things are going badly wrong.
Ever since Mr Putin announced in September that he was coming back, the mood has gradually changed. The first sign was when he was whistled as he addressed the crowd at a stadium after a martial arts fight last month. Then, last week's elections. Everyone has always suspected that totals for United Russia are artificially inflated, and that many of those who vote for the party are forced to do so. This time, though, people got angry. They were the first countrywide elections since young Russians started using Facebook and Twitter, and suddenly the evidence of falsifications was there for all to see.
The Kremlin response has been familiar. Protests have been violently broken up; protest websites have been hacked, and organisers intimidated. Amazingly, none of it worked. Young, middle-class Russians, who for years have enjoyed the fruits of the Putin-era oil boom and have turned a blind eye to politics, have suddenly started taking an interest. Authorities watched with horror as the number of people who said they would attend Saturday's protest grew steadily throughout the week.
The fact that the protest was "sanctioned", that a cordon of riot police was set up to allow protesters to march through central Moscow, and that there was no police violence, suggests that somebody high up realises that while a protest of 200 people can be dispersed with arrests and fear tactics, a protest of 30,000 cannot be.
Mr Putin will still almost certainly win the presidential election in March. But the sudden protest mood cannot be ignored, and if it does not peter out, the Kremlin will face a difficult choice. Does it open up the political process, allowing real criticism and scrutiny of Mr Putin and those around him, which could seriously dent his popularity, or does it repress dissent and risk radicalising the opposition? Either option has obvious pitfalls. The only certainty is that Russian politics has entered a new phase.