Russian authorities on Friday declared opposition politician Vladimir Kara-Murza a "foreign agent" and ordered his pre-trial detention for allegedly spreading false information about the Russian army amid its military campaign in Ukraine.

Moscow's Basmanny district court ordered that the 40-year-old Kremlin opponent be held in jail until June 12, his lawyer Vadim Prokhorov said on Facebook.

Prokhorov said earlier in the day that Kara-Murza was facing criminal charges over spreading allegedly false information.

The Western-educated activist and journalist was a close associate of opposition leader Boris Nemtsov, who was assassinated near the Kremlin in 2015, and Mikhail Khodorkovsky, a former oligarch turned Kremlin critic.

He has for years campaigned in the West against President Vladimir Putin's regime.

According to a copy of an order on institution of criminal proceedings posted by Kara-Murza's lawyer on Facebook, the case against him was launched over his speech about Putin's military campaign in Ukraine to members of the lower house of the Arizona Legislature in mid-March.

Asked about the criminal charges, the Kremlin on Friday told reporters that "there is a law that is being implemented".

"The Investigative Committee exercises its powers and duties in accordance with this law," Kremlin spokesman Dmitry Peskov said.

The law, which was introduced after Putin sent troops to Ukraine on February 24, imposes prison sentences of up to 15 years for publishing information about the military deemed false by the Russian government.

Kremlin opponent Vladimir Kara-Murza, pictured in October 2021, has been ordered held in jail until June 12
Kremlin opponent Vladimir Kara-Murza, pictured in October 2021, has been ordered held in jail until June 12 AFP / Alexander NEMENOV

Last week, Kara-Murza was sentenced to 15 days in jail for disobeying police orders.

Kara-Murza claims he was poisoned twice -- in 2015 and 2017 -- because of his political activities.

In a separate move, Kara-Murza was declared a "foreign agent" along with several prominent journalists including Alexei Venediktov, the editor of the now dismantled Echo of Moscow radio station.

The Russian justice ministry said in a statement that it had added to its growing list of "foreign agents" eight individuals including Leonid Volkov, the right-hand man of Alexei Navalny, the jailed leader of the Russian opposition, and veteran journalist Alexander Nevzorov.

In March, Russian investigators launched a criminal case against Nevzorov for alleging that Moscow's army deliberately shelled a maternity hospital in Ukraine's embattled city of Mariupol.

Moscow has stepped up efforts to stamp out dissent after Putin sent troops to Ukraine.

Independent media outlets have been shut down or suspended operations, and tens of thousands of Russians have left the country in protest over the Kremlin's policies.

By law, individuals or entities identified as "foreign agents" must disclose sources of funding, undergo audits and accompany all their texts, videos and social media posts with a caption mentioning content from a "foreign agent".

The status is reminiscent of the Soviet-era term "enemy of the people" and is meant to apply to people or groups that receive funding from abroad and are involved in any kind of "political activity".