Russian investigators on Wednesday charged jailed opposition leader Alexei Navalny with a new crime that could prolong his time behind bars by up to three years.

President Vladimir Putin's most vocal domestic critic is in prison serving two and a half years on old fraud charges after returning from Germany once he recovered from a near-fatal poisoning attack.

The Russian Investigative Committee -- which probes major offences -- charged him with the additional crime of "creating a non-profit organisation that infringes on the identities and rights of citizens."

It said Navalny's FBK anti-corruption foundation -- branded as extremist earlier this summer -- "encouraged citizens to commit illegal acts".

Investigators also said Navalny had called on Russians to take part in unsanctioned rallies while "aware" of their "unlawful nature".

If found guilty, Navalny could face an extra three years in prison and could only be released after 2024, the year Russia is scheduled to hold a presidential election.

Navalny's movement is facing unprecedented pressure in Russia ahead of parliamentary elections in September, with FBK and its regional offices outlawed.

Calls for Smart Voting

The Kremlin critic's team slammed the charges as "another stupid accusation."

"Nobody infringes on the identity and rights of citizens like Putin himself and all his henchmen," it said on social media.

It called on Russians to vote against the ruling United Russia party next month to reject its "endless power".

Writing on Telegram, it encouraged Russians to use Navalny's "Smart Voting" strategy that calls on voters to back candidates best placed to defeat Kremlin-linked politicians.

Russia's opposition says authorities have stepped up a campaign of intimidation against dissenters ahead of the vote, allegations the Kremlin has rejected.

The new charges came a day after investigators launched a new criminal case against Navalny's key allies Leonid Volkov and Ivan Zhdanov over raising funds for an "extremist" organisation.

Both Volkov and Zhdanov have left Russia, under pressure from authorities. The charges against them carry a maximum penalty of eight years in jail.

This week a Moscow court dropped a libel lawsuit filed by Navalny in late March against Kremlin spokesman Dmitry Peskov over comments suggesting the opposition figure has links with the CIA.

Last week, Navalny's brother Oleg was handed a one-year suspended sentence for breaching coronavirus rules by calling on Russians to attend an unsanctioned rally. Key ally Lyubov Sobol was sentenced to a year and a half of restricted movement in the same case.

Russia's state censor in July blocked nearly 50 Navalny-linked websites and allies say the media watchdog is also seeking to suspend their social media channels.