Russian authorities have opened a third criminal investigation into opposition leader Alexei Navalny, saying the critic of President Vladimir Putin is suspected of stealing millions of dollars from a political party in 2007.

Navalny, 36, the most prominent leader of large street protests that erupted last December against Putin's nearly 13-year rule, already faces up to 10 years in jail if convicted on existing charges of theft from a state timber company.

Earlier this month, investigators also charged Navalny and his brother of cheating a mail-transport company out of US$1.79 million (NZ$2.18m). That move followed a December 15 opposition rally at a memorial to victims of Soviet-era repressions outside the headquarters of the FSB security service, formerly the KGB.

Navalny, a lawyer and anti-corruption blogger, denies any wrongdoing and says the accusations aired by the Investigative Committee, a federal agency that answers to Putin, are intended to persuade him to stop his opposition activities.

"Aie, aie, aie, yet another case against me. Investigative Committee, what are you doing? ... That's enough," Navalny tweeted minutes after the new investigation was announced.

The Interfax news agency quoted him as saying the charge was "absolutely absurd".

On its website, the Investigative Committee said it suspects an advertising company headed by Navalny stole up to 100 million roubles (NZ$3.94m) paid to it by the liberal, pro-business Union of Right Forces Party (SPS) for campaign advertising.

It said there was evidence Navalny's company, Allekt, siphoned the money off into bank accounts of shell companies.

A former SPS party leader, Leonid Gozman, dismissed the charges as nonsense and said the party had no accusations or complaints against Navalny, Interfax reported.

Navalny said the fact investigators had initiated the case without a complaint from the party showed it was politically motivated, and he vowed not to give up his activism, Interfax said.

The Investigative Committee said it had encountered the evidence related to SPS while investigating its case against Navalny on the timber theft charge.

Navalny is a key leader of an opposition council, elected in an online vote in October, which is trying maintain the momentum of a protest movement that drew crowds of up to 100,000 people onto Moscow's streets at its peak last winter, but failed to prevent Putin winning a six-year third term as president.

Since then, pro-Kremlin lawmakers have passed a series of laws that the opposition say are intended to stifle dissent. Several opposition leaders and dozens of activists are facing criminal charges or investigation.

- Reuters