Opposition leaders Sergei Udaltsov and Leonid Razvozzhayev were charged formally by Russian prosecutors with plotting major unrest on the eve of President Vladimir Putin's inauguration in 2012 for his third non-consecutive term.
The Russian Investigative Committee said both men were charged but refused to testify after issuing not guilty pleas.
"Both Razvozzhayev and Udaltsov were questioned, both did not admit their guilt and refused to give testimony," a committee statement published Thursday by RIA Novosti said. "The investigation of this criminal case continues."
Putin's re-election was marred by protests and his administration has cracked down on political opponents. His administration defended a controversial law targeting non-governmental organization as part of a widespread transparency initiative.
Udaltsov and Razvozzhayev are suspected of plotting against the state with Georgian leaders.
A dozen opposition figures are on trial for their role in May 2012 demonstrations.