A Russian opposition politician, Alexei Navalny, will face an additional 19 years imprisonment on charges of extremism that are widely considered to be bogus, underscoring the perils of openly criticizing the Kremlin while Vladimir Putin remains president of Russia.

A Russian court convicted Mr. Navalny, who is already serving a nine-year term on a variety of charges that he says were politically motivated, in a criminal case which he said afterward was designed to make ordinary Russians fear the Kremlin’s authority.

The new charges are related to the activities of Mr. Navalny’s anti-corruption foundation and statements by his top associates. It was his fifth criminal conviction and his third and longest prison term — all of which his supporters see as a deliberate Kremlin strategy to silence its most ardent opponent.

Yet he has not been silenced. 

Mr. Navalny appeared in the courtroom wearing prison garb and looking gaunt, but with a defiant smile on his face. As the judge read out the verdict, the politician stood alongside his lawyers and his co-defendant with his arms crossed, listening with a serious expression on his face.

In a statement on social media released via his lawyers and supporters, Mr. Navalny said, “I understand perfectly well that, like many political prisoners, I am serving a life sentence. Where the life sentence is measured by the length of my life or the length of the life of this regime.”

He urged resistance to that regime: “You, not me, are being frightened and deprived of the will to resist. You are being forced to surrender your Russia without a fight to the gang of traitors, thieves and scoundrels who have seized power. Putin must not achieve his goal. Do not lose the will to resist.”

Russian state news agencies said that Mr. Navalny would serve the new term concurrently with his current sentence on charges of fraud and contempt of court. 

The trial took place behind closed doors in the penal colony east of Moscow where Mr. Navalny is currently  imprisoned.

Mr. Navalny’s spokeswoman, Kira Yarmysh, told the AP that the politician feels optimistic despite the harsh sentence, and “absolutely believes in what he’s doing,” adding that “it certainly helps him cope with all that and keep doing what he’s doing.”

The Department of State condemned Mr. Navalny’s new sentence as “an unjust conclusion to an unjust trial” and called for his immediate release.

“For years, the Kremlin has attempted to silence Navalny and prevent his calls for transparency and accountability from reaching the Russian people,” it said. “By conducting this latest trial in secret and limiting his lawyers’ access to purported evidence, Russian authorities illustrated yet again both the baselessness of their case and the lack of due process afforded to those who dare to criticize the regime.”

The 47-year-old Mr. Navalny is Mr. Putin’s fiercest foe and has exposed official corruption and organized major anti-Kremlin protests. He was arrested in January 2021 upon returning to Moscow after recuperating in Germany from nerve agent poisoning that he blamed on the Kremlin.

Mr. Navalny’s allies said the extremism charges retroactively criminalized all of the anti-corruption foundation’s activities since its creation in 2011. In 2021, Russian authorities outlawed the foundation and the vast network of Mr. Navalny’s offices in Russian regions as extremist organizations, exposing anyone involved to possible prosecution.

The UN human rights chief, Volker Türk, said Mr. Navalny’s new sentence “raises renewed serious concerns about judicial harassment and instrumentalisation of the court system for political purposes in Russia” and called for his release.

One of Mr. Navalny’s associates, Daniel Kholodny, stood trial alongside him after being relocated from a different prison. His lawyer told the independent Novaya Gazeta newspaper that Mr. Kholodny was sentenced to eight years in prison.

Mr. Navalny rejected all the charges against him as politically motivated and accused the Kremlin of seeking to keep him behind bars for life.

On the eve of the verdict hearing, Mr. Navalny released a statement on social media, presumably through his team, in which he said he expected his latest sentence to be “huge … a Stalinist term.” Under the Soviet dictator, Joseph Stalin, millions of people were branded “enemies of the state,” jailed, and sometimes executed in what became known as the “Great Terror.”

In his statement, Mr. Navalny called on Russians to “personally” resist and encouraged them to support political prisoners, distribute flyers, or go to a rally. He told Russians that they could choose a safe way to resist, but he added that “there is shame in doing nothing. It’s shameful to let yourself be intimidated.”