MOSCOW — Russian authorities arrested one of the country’s most prominent political opposition leaders on Wednesday, accused him of plotting to organize mass riots, and said he could face terrorism charges.
The arrest of Sergei Udaltsov, the buzz-cut, black-clad leader of the Left Front, a radical socialist group, seemed to accelerate the government’s efforts to bring serious criminal cases — with the prospect of long prison terms — against critics of President Vladimir V. Putin.
Udaltsov, a fixture at antigovernment rallies, is arrested frequently and has served numerous short stays in prison, often for administrative offenses like disobeying the police.
But the new accusations are far more serious, stemming from a recent documentary on the progovernment NTV channel that appeared to show him discussing efforts to topple the Russian government with an official from neighboring Georgia, as well as appealing for financial support.
A spokesman for Russia’s top federal investigator, Aleksandr Bastrykin, even raised the prospect of life imprisonment.
‘‘I would like to draw the attention of those who thought that in our country it is possible with absolute impunity to organize mass disorders, to plan and prepare terrorist acts and other actions threatening the life and health of Russians,’’ the spokesman, Valery Markin, said. ‘‘You underestimate the professionalism of Russia’s special services.’’
He added that Russian law ‘‘envisages for such offenses punishment up to life imprisonment.’’
Although many of the best-known political opposition leaders in Russia have come under pressure of criminal investigations recently— notably, corruption fighter Aleksei Navalny and television personality Kseniya Sobchak — there has yet to be a major prosecution.
Udaltsov poses a particular threat to the Kremlin because his views are popular among members of the Communist Party, which retains a strong nationwide infrastructure and often finishes second to the governing party, United Russia, in major elections.