Russia's jailed opposition leader Alexei Navalny has urged thousands of supporters to turn out for nationwide rallies on President Vladimir Putin's birthday on Saturday to protest against his nearly two-decade rule.

The charismatic anti-corruption campaigner who has declared his intention to run in a presidential election next March to unseat Putin, is hoping to see protests in some 80 Russian cities with the main rallies in Putin's hometown of Saint Petersburg and the capital Moscow.

In an address dictated from his prison cell, the 41-year-old Yale-educated lawyer, with a street-smart image and a penchant for catchy slogans, compared life under Putin's regime to a forced diet of "turnip."

"If we do nothing, they will be feeding us this damn turnip for the rest of our lives. And our children too," Navalny said this week.

A court on Monday sentenced him to 20 days in jail on charges of repeatedly violating a law on organising public meetings.

Contested elections along with Navalny's release from jail and permission for him to put his name on the ballot are the opposition's top demands.

The authorities in Moscow and Saint Petersburg have refused to allow the protesters to gather in the city centre, and the rallies could end in violence.

In a strongly-worded video address released by Navalny's campaign team, one of Russia's most acclaimed film directors, Andrei Zvyagintsev, slammed Putin for hindering Navalny. 

He criticised the prospect of Russians voting in polls where "we are asked to choose one out of one."

"It's just revolting watching this spectacle," he said.

Putin, who turns 65, has ruled over Russia since 1999. He said this week he has not yet decided whether he will seek another six-year term. But he is widely expected to run in March elections and win.

Navalny brought tens of thousands of supporters -- many of them students and schoolchildren -- onto the streets for unauthorised protests across the country on March 26 and June 12.

The participation of minors in opposition rallies stunned the authorities, with the protests ending in violent clashes.

Police arrested more than 1,000 people in Moscow alone at the March 26 demonstration.

- 'Nothing to lose' - 

Navalny himself was detained even before arriving at those two rallies in Moscow and served 15 and 25 days in jail for organising the events. Several protesters were given lengthy prison terms.

Navalny has been able to breathe new life into the protest movement by tapping into the anger of Russia's young generation who have grown up under Putin and yearn for change.

Polina Kostyleva, the head of the opposition politician's campaign headquarters in Saint Petersburg, was hard-pressed to say how many people would turn out in the city on Saturday after the latest series of arrests and searches targeting Navalny and his supporters.

"Anything can happen. Young people show up -- they have nothing to lose," she told AFP.

"This event will show the authorities' attitude towards Navalny and us." 

Navalny was arrested last week as he was planning to travel to a rally in a provincial city, part of his election campaign that has seen him rouse supporters across the country.

One of his closest allies, Leonid Volkov, has also been sentenced to 20 days in jail.

In the past days, Navalny's Moscow campaign headquarters and several other offices have been raided.

Officials have said he is not eligible to run for president because he is serving a suspended sentence for fraud.

Navalny shot to prominence as an organiser of huge anti-Putin rallies that shook Russia in 2011-2012 following claims of vote-rigging in a parliamentary poll but gradually died down.

Navalny stood for Moscow mayor in 2013 with a Western-style campaign and a message of snuffing out corruption, coming second against a Kremlin-backed incumbent.

Kremlin spokesman Dmitry Peskov said Putin would celebrate his birthday with his family and also chair a meeting of Russia's security council.

Officials and Kremlin supporters seek to outdo one another as they sing Putin's praises on his birthday.