To describe Alexei Navalny's trial as "behind closed doors" is an understatement. It is being held in a high-security prison.

Penal Colony No 6 is 240km (150 miles) east of Moscow. Navalny, the Kremlin's most vocal critic, is already serving a nine-year term there for parole violations, fraud and contempt of court: charges widely seen as politically motivated.

His time behind bars looks set to be extended.

In June, a hall in the penal colony was turned into a makeshift courtroom and Navalny was back in the dock. The charges against him this time include creating an extremist organisation and financing extremist activities.

Russian state prosecutors have called for a 20-year prison sentence for Navalny in an even more restrictive "special regime colony". Such prisons are normally reserved for Russia's most dangerous criminals.

In a message posted on social media ahead of the verdict, which is due on Friday, Navalny expressed his belief that he would be handed "a big [prison] term".

We were among a group of journalists allowed briefly into the penal colony for the start of the trial. In a "press room" we could view proceedings on a video screen.

Navalny had clearly lost a lot of weight in prison. But he was defiant as he railed against the judge and his trial behind bars.

On paper, it is a Moscow court that is hearing the case. But the decision to hold a remote trial in a prison suggests that the Russian authorities wanted to avoid the publicity that would inevitably come with transporting Navalny to the Russian capital.

The picture was not on the screen for long. An hour and a half into the trial, the prosecutor demanded the closing of proceedings to press and public. The judge agreed. The video feed was cut.

That has made it difficult to follow what has been happening in this trial.

When Navalny delivered his final statement two weeks ago, there was no video or audio recording of his words. But the text of his speech criticising the Russian authorities and the war in Ukraine was made public. His supporters, including Russian actors and musicians in exile, read it out and posted online.

Even when this case is over, there may be more to follow. Navalny says that investigators have told him to expect another trial: on terrorism charges.

Over the years Vladimir Putin's Kremlin has been busy removing all potential rivals to the president, clearing the Russian political landscape of likely challengers. It will want to make sure that its loudest critic stays well away from Russia's political stage.

For more than a decade, Navalny has exposed corruption at the heart of Russian power. His video investigations have received tens of millions of views online.

But it is his ability to mobilise the public, especially young Russians, to take to the streets which makes the authorities especially nervous. In recent years Navalny has been the only Russian opposition leader capable of organising anti-Putin protests on a national scale.

He had set up a network of regional campaign offices, having planned to run for president in 2018. But he was barred from the vote. The authorities have already declared Navalny's network of offices and his Anti-Corruption Foundation "extremist" and shut them down.

In 2020, Navalny was poisoned in Siberia by what Western laboratories later confirmed to be a nerve agent. He later accused the Kremlin of trying to kill him. The Russian authorities deny that.

After he received urgent medical care in Germany, his decision to return to Russia in 2021 will have been viewed by those in power here as a direct challenge to the Kremlin. He was arrested on arrival at a Moscow airport.

"Navalny is a symbolic figure. And the Kremlin is afraid of him," believes Andrei Kolesnikov of the Carnegie Russia Eurasia Centre. "Even though, as polls show, the average Russian can't see Navalny right now. He's off the information field. But for the Kremlin that doesn't matter. It still views him as an enemy and a danger.

"The regime is ready to be extremely cruel. It sends messages to the broader audience: We will not stop. The machine is working and doesn't have a reverse gear. It means that they are ready to continue all possible trials against all possible dissidents."

And not just against pro-democracy, anti-war figures like Navalny.

Last month Russian nationalist Igor Girkin (also known as Igor Strelkov) was arrested in Moscow on extremism charges.

Mr Girkin not only supports Russia's war in Ukraine - in 2014, the retired FSB security officer organised and commanded pro-Russia militias in Ukraine's Donbas region - he even boasted that he had "pulled the trigger of this war".

Last year he was convicted by a court in The Hague in absentia for his role in the shooting down of Malaysia Airlines flight MH17.

In his more recent role of nationalist blogger, he had become increasingly critical of the Russian authorities' handling of the war - and of President Putin himself.

In one post he called the Kremlin leader "a non-entity" and "a cowardly waste of space".

"It is not enough to support the war," Andrei Kolesnikov says. "You must do it in the right way. You must do it while supporting Putin's narrative, his ideas and goals. You can't criticise Putin.

"This is an authoritarian and partly totalitarian regime. They can't stop themselves. They must control everything. They must suppress everything.

"At the end of the day, they are afraid of any kind of resistance, any alternative leadership."