On Saturday, Russian President Vladimir Putin will mark his 65th birthday. This is five years past the retirement age for men in Russia – and roughly their life expectancy too – but there is little prospect of an exit for a man in his 18th year of power.

One man aiming to spoil Mr Putin’s party is Alexei Navalny, the Russian opposition leader who has made clear his desire to run for the presidency in elections next year – even if the Kremlin has said he is ineligible.

To mark the occasion, Mr Navalny, 41, has called on supporters to rally in more than 80 Russian cities, including the president’s home city of St Petersburg. Mr Navalny, who was planning to attend the St Petersburg rally, instead lost an appeal against his 20-day jail sentence imposed by a Moscow court for calling the unsanctioned rally.

Authorities in Moscow and St Petersburg have not granted permission for rallies in those cities, warning people will face arrest if they attend.

For Mr Putin, the rallies will be seen as more of an inconvenience for now. In September, he equalled Leonid Brezhnev’s time in office – 6,602 days – leaving him closing in on Stalin’s record secure in the knowledge his support base is holding strong.

Mr Putin has so far been cagey about his plans. Last week, the President said that he might not, in fact, run in presidential elections next March.

But few in Moscow believe a serious operation to install a successor is under way. None of the tea leaves read that way. The reason for such suspense is most likely suspense alone: how else to make one of the most boring referendums on earth exciting?

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On Friday, TV Rain, one of the last remaining liberal voices in Russia, reported that Mr Putin’s presidential run will now be announced only at the very last moment. Citing high-placed sources in the President’s administration, the channel said that the date had been put back as far as possible to limit any chance of other candidates gaining momentum.

“The later the campaign, the less time opponents have for their own PR, and the better it becomes for the main candidate,” the Kremlin source is quoted as saying.

According to Russian law, the official campaign needs to begin by 7 December, or three months before election day. So that would mean we should expect Mr Putin’s announcement at the end of November or first week in December.

But, in the time that Mr Putin’s increasingly narrow circle of advisers were considering the best way to give their man a clear run, his most obvious challenger had, in fact, built improbable momentum. In the space of a few short months, Mr Navalny has built an impressive grassroots movement across Russia.

If not quite in shape to directly challenge Mr Putin in next year’s elections, a reasonable result for Mr Navalny – anything above 15 per cent – would establish his legitimacy as a politician. From there, the future is anyone’s guess.

Putin watches Russia's military might on display in war games

But for the Kremlin, the issue of Mr Navalny running next March is already closed. A criminal conviction that most consider politically motivated rules him out of running, they say.

After allowing several rallies in September, last week authorities increased pressure on the uninvited presidential candidate. As well as Mr Navalny, his chief of staff, Leonid Volkov, was also sentenced to 20 days arrest for organising “unsanctioned” rallies. At the sentencing, Mr Navalny said that President Putin had given himself a “birthday present” to try and ensure the St Petersburg rally did not go ahead.

Mr Navalny’s team have said the programme of rallies will go ahead regardless, adding dozens of other cities. Large crowds are expected in Moscow and St Petersburg in particular, despite the warnings from city officials about the threat of arrest.

The police response is anyone’s guess. There has been a tendency to go soft when least expected, and the contrary. But the last time a large protest coincided with a big day for Mr Putin – his inauguration weekend in 2012 – the response was uncompromising. Hundreds of arrests were made.

The Kremlin has only used a fraction of its political resources on containing Mr Navalny, but a source close to the Kremlin confirmed concern about his ability to connect with young voters remains. Mr Navalny’s taunts of “Grandad Putin” are directly aimed at Putin’s electoral appeal, which has traditionally focussed on youth and energy.

Of course, much has changed since the last election. With falling real-terms income, and complicated international relations, Mr Putin is instead usually presented as a national leader.

But the focus on youth, strangely enough, remains. Commentators in loyal newspapers are already promoting the idea of Mr Putin’s “third youth”. And if not his own, then of others. Last week, eight mostly long-serving governors were replaced with young technocrats. This follows a similar process last year. The week before, Mr Putin visited Yandex, Russia’s answer to Google, with the tech giant under strict instructions that the President at all times be surrounded by young employees.