Udaltsov, coordinator of the Left Front movement, told reporters outside the Investigative Committee offices in Moscow Friday the case against him was "a lie and a provocation," RIA Novosti reported.
"If they expected me to leave the country, it was in vain. I will bear all of it and I hope those who support me will not be silent or give in," he said.
If found guilty, he could be imprisoned for up to 10 years.
Udaltsov, 35, said on his Twitter page the regime of President Vladimir Putin "will be on trial."
He was released on a pledge he wouldn't leave Moscow, RIA Novosti said.
His lawyer, Violetta Volkova, earlier said she thought her client would be taken into custody.
"The investigators are playing some kind of game," Udaltsov tweeted on Friday.
The case against Udaltsov was opened after an Oct. 5 broadcast of "Anatomy of a Protest 2," the second chapter of an expose on the opposition aired by NTV, a state-owned and pro-Kremlin television network, RIA Novosti said. The report purported to show Udaltsov and two other opposition figures, along with a Georgian politician, plotting "mass disorder."
Udaltsov was a key leader in protests that began in Russia after parliamentary elections in December.