Udaltsov and his counterpart, 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.
A court in Moscow ruled against a defense challenge to his detention, ruling his house arrest was necessary, state news agency RIA Novosti reported Wednesday.
Putin's election was marred by protests. His administration has been charged by human rights groups for cracking down on his opponents.
Human Rights Watch said Wednesday the Russian government was using questionable laws to silence its critics. It said two civic organizations were targeted for not registering as "foreign agents."
"We've seen that the authorities' definition of 'political' includes activities that are a routine part of advocacy work," Rachel Denber, a research director at Human Rights Watch, said in a statement. "The aim here is to demonize independent critics as agents of foreign interests."
Russia in 2012 enacted legislation that requires civic organizations to register as "foreign agents" if they receive some of their funding from outside sources.