Kremlin critic Alexei Navalny has lost an appeal against what he says was a politically motivated decision to jail him for nearly three years, but his prison term has been slightly shortened.

Mr Navalny, President Vladimir Putin's most prominent critic, was jailed earlier this month for parole violations that he said were trumped up.

Western countries have condemned the case and are discussing possible sanctions on Russia.

A Moscow court swiftly rejected his appeal, while shortening his original jail term by six weeks.

The original sentence was three-and-a-half years.

But, with the amount of time he had already spent under house arrest taken into account, it amounted to around two years and eight months.

His lawyer said on Saturday he would now spend a little over two-and-a-half years behind bars.

Mr Navalny responded sarcastically to the ruling.

"They've reduced the sentence by one-and-a-half months. Great!" he said from a courtroom glass cage.

The opposition politician had earlier told the judge he was not guilty of parole violations as a previous court had found.

Navalny quotes pop culture

Speaking before the verdict, Mr Navalny referenced the Bible as well as Harry Potter and the animated sitcom Rick and Morty as he urged Russians to resist pressure from the authorities and challenge the Kremlin to build a fairer and more prosperous country.

"The government's task is to scare you and then persuade you that you are alone," he said.

"Our Voldemort in his palace also wants me to feel cut off," he added, in a reference to Mr Putin.

"To live is to risk it all.

"Otherwise, you're just an inert chunk of randomly assembled molecules drifting wherever the universe blows you."

Mr Navalny also addressed the judge and the prosecutor, arguing they could have a much better life in a new Russia.

"Just imagine how wonderful life would be without constant lying," he said.

"Imagine how great it would be to work as a judge when no-one would be able to call you and give you directions on what verdicts to issue."

Mr Navalny returned to Russia last month from Germany, where he had been recovering from a near-fatal poisoning in Siberia in August with what many Western nations said was a nerve agent.

He said he had been unable to report to the Moscow prison service last year because he had been convalescing in Germany at the time.

"I don't want to show off a lot, but the whole world knew where I was," Mr Navalny told the judge.

"Once I'd recovered, I bought a plane ticket and came home."

Mr Navalny said he had no regrets about returning to Russia, that his belief in God helped sustain him, and that "strength was in truth".

"Our country is built on injustice. But tens of millions of people want the truth. And sooner or later they'll get it," he said.

Back at court for slander case against war veteran

Mr Navalny is due to appear in court again later on Saturday for what is expected to be the culmination of a separate slander trial against him.

In the slander case, Mr Navalny has been accused of defaming a World War II veteran who took part in a promotional video backing constitutional reforms last year that let Mr Putin run for two more terms in the Kremlin after 2024 if he wants.

Mr Navalny described the people in the video as traitors and corrupt lackeys.

But he has said his comments were not specifically directed against the veteran, and that the authorities are using the charge to smear his reputation.

State prosecutors have asked the court to fine Navalny 950,000 roubles ($16,300) for slander.

Mr Navalny's arrest and jailing sparked nationwide street protests in Russia, but his allies say they have now paused major demonstrations until the spring.

Mr Navalny accuses Mr Putin of ordering his attempted murder.

Mr Putin has dismissed that, alleging Mr Navalny is part of a US-backed campaign to discredit him.

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ABC/Wires