Prosecutors have requested a 25-year prison sentence for Kremlin critic Vladimir Kara-Murza, his lawyer said, adding that the case against the Russian opposition activist on several charges including treason for comments critical of Russia's invasion of Ukraine is proceeding quickly.

Kara-Murza, who was detained in April 2022 after returning to Russia from abroad, is the latest in a string of opposition activists, reporters, and others who have been arrested and prosecuted amid a growing Kremlin crackdown on civil society.

His lawyer, Maria Eismont, told journalists that the prosecutor's request for the 25-year sentence came at a closed hearing on April 6.

Eismont added that the 41-year-old Kara-Murza, who twice nearly died after what he says were deliberate poisoning attacks, has lost 17 kilograms during his detention.

The trial of the prominent activist was expected to open on March 16 but was delayed until March 20 after his lawyer told the court his health had "significantly deteriorated."

A certificate from the medical unit of Kara-Murza's detention facility read by the judge during a hearing last month said that he could not participate in the trial "until the end of the current course of treatment" being prescribed.

It added that he was being treated for polyneuropathy, which he says is a result of the poisonings.

Prosecutors initially charged him with disobeying a police officer but later added a new allegation of discrediting the Russian military, a charge stemming from Russia's 2022 invasion of Ukraine and a Kremlin push to stamp out criticism or dissent on the subject.

State treason charges were later added, with prosecutors citing remarks he made in speeches outside of Russia that criticized Kremlin policies.

Аt a preliminary hearing in Moscow City Court on March 13, which was ordered held behind closed doors, Kara-Murza entered a not-guilty plea, according to his lawyers.

A top deputy to former Deputy Prime Minister Boris Nemtsov, who was gunned down near the Kremlin in 2015, Kara-Murza had cultivated ties and support with Western lawmakers and policymakers, particularly in the United States.

The late U.S. Senator John McCain was a proponent of Kara-Murza's efforts, and he served as a pallbearer at McCain's funeral in 2018.

Kara-Murza fell ill twice while traveling in Russia in two different situations, first in 2015 and then again in 2017. In both cases, he was hospitalized in critical condition but eventually recovered.

His family members brought tissue samples out of Russia for independent testing to try and determine what was behind the sudden illnesses.

The FBI investigated the case as "intentional poisoning" and enlisted its crime laboratories and some of the federal government's elite scientific laboratories to examine the samples.

The effort was also unusual in that it drew the attention and involvement of FBI Director Christopher Wray and members of the White House National Security Council.

With reporting by AFP and Reuters