Russian opposition politician Alexei Navalny expects a court to extend his prison sentence by nearly two decades on Friday, in a criminal case which he and his supporters say was trumped up to keep him behind bars and out of politics for even longer.

Navalny, 47, President Vladimir Putin's fiercest domestic critic, is already serving sentences totalling 11-1/2 years on fraud and other charges that he says are also bogus. His political movement has been outlawed and declared "extremist".

State prosecutors have asked the court to hand him another 20 years in a penal colony on six separate criminal charges, including inciting and financing extremist activity and creating an extremist organisation.

In a message posted on social media on Thursday, Navalny said the outcome could be slightly less, around 18 years, but it didn't really matter because he was also threatened with terrorism charges that could bring another decade.

"It's going to be a long sentence. What is called 'Stalinist'," said Navalny, who is able to post on social media via his supporters and lawyers.

He said the purpose would be to frighten Russians, but urged them not to let that happen and to think hard about how best to resist what he called the "villains and thieves in the Kremlin".

The charges relate to his role in his now defunct movement inside Russia, which the authorities said had been trying to foment a revolution by seeking to destabilise the socio-political situation.

'Sacrifices'

In his closing statement last month, delivered behind closed doors at the IK-6 penal colony in Melekhovo, about 235 km (145 miles) east of Moscow where he is serving his sentences, Navalny explained why he would keep opposing the Russian authorities.

"For a new, free, rich country to be born, it must have parents. Those who want it. Who expect it and who are willing to make sacrifices for its birth," Navalny said, according to a text supplied by his aides.

Putin, in power since 1999, is expected to run for another six-year presidential term in 2024. With Russia waging what he calls its "special military operation" in Ukraine and locked in what he describes as an existential battle with the West, Putin says it is vital for the country to remain united.

In February, Putin ordered the FSB security service to raise its game and said it was necessary "to identify and stop the illegal activities of those who are trying to divide and weaken our society".

Navalny, who in the 2010s brought tens of thousands of people onto the streets, was detained in January 2021 after returning to Moscow from Germany where he had been treated for what Western doctors said was poisoning by a Soviet-era nerve agent.

The Kremlin, which at one point accused him of working with the CIA to undermine Russia, denied any involvement in what happened to him and denies persecuting Navalny. It has portrayed him as an agent of disruption and says he never represented serious political competition.

The Kremlin says his case is purely a legal matter for the courts. His supporters cast Navalny as a Russian version of South Africa's Nelson Mandela who will one day be freed from prison to govern the country.