MOSCOW — Russian authorities on Wednesday arrested one of the country’s most prominent political opposition leaders, 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 sharply accelerate the government’s efforts to bring serious criminal cases — with the prospect of long prison terms — against critics of President Vladimir V. Putin.

Mr. Udaltsov is a fixture at antigovernment rallies, is arrested frequently and has had 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 pro-government 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.