Sergei Udaltsov, leader of the Left Front group, was in a Moscow hospital for much of his detention as a result of the health effects of a hunger strike.
Two Russian activists who challenged officials last month amid outcry over disputed State Duma elections have been released after serving their jail sentences.
Both arrests came amid demonstrations against the December 4 parliamentary elections that sparked some of the biggest street protests in Russia since the collapse of communism.
One of the detainees, 34-year-old Left Front leader Sergei Udaltsov, was jailed shortly after the voting and initially sentenced to five days of detention. A series of police and court actions extended his imprisonment to a full month, much of which he spent in hospital after a hunger strike threatened his health. He has called his arrest "unlawful."
The other released activist is Yaroslav Nikitenko, the deputy head of a group fighting a Moscow-to-St. Petersburg highway project that runs through the Moscow district's Khimki Forest.
Nikitenko was detained at a late-December rally in support of Udaltsov and also released on January 4, after serving a 10-day sentence.
The Left Front's Udaltsov said after his release that he was "weakened" physically but that his condition was improving, according to RFE/RL's Russian Service.
He told supporters to keep up the fight against Prime Minister Vladimir Putin's hold on power and to work for a democratic Russia. Udaltsov urged people to turn up for a peaceful pro-democracy march on February 4.
WATCH: Sergei Udaltsov is greeted by scores of supporters after his release from detention in Moscow late on January 4:
Your browser doesn’t support HTML5
Political Activist Udaltsov Freed
"I feel drunk with freedom, and I feel that people support me -- not only me, but the whole idea that we are fighting for," Udaltsov told about 100 supporters who were on hand for his release, according to wire reports. "The support of a huge amount of people inspires me and gives me strength."
Putin, who is the front-runner for a return to the Kremlin after a scheduled presidential election in March, and President Dmitry Medvedev have defended the December vote and resisted opposition demands that it be repeated because of allegedly widespread fraud.
International monitors also registered voting irregularities, including so-called carousel voting, in which groups of people are transported to a number of polling places to vote multiple times.
Udaltsov called the current atmosphere in Russia "one of the most emotional times of my life."
"But I hope that the most emotional moment will be when we achieve our results that are being discussed right now by hundreds of thousands, millions of people in this country," Udaltsov was quoted as saying, "when we finally liberate Russia and make a fair, prosperous country."
"Right now, those in power are waiting," Reuters quoted Udaltsov as saying. "They hope that our protest, that wave, will sleep. I, for one, am sure that it's not like that. It's not the fashion of the moment. This is really that society woke up and no longer tolerated unlawful humiliation. For that reason, I think that we finish our business and force those in power to grant our absolutely legal demands or leave the political stage."
Nikitenko was detained on December 25 at a rally in support of Udaltsov.
But official harassment and even brutal attacks have targeted activists and journalists involved in the defense of the Khimki woodlands. A number of Nikitenko's colleagues from the Defenders of Khimki Forest have been arrested, and they have accused authorities of abusing agencies like Russia's Child Protective Service to target their children.
Fellow environmental activist Yelena Nadezhkina told RFE/RL's Russian Service that Nikitenko was detained by police while "just standing there and tweeting" on events at the pro-Udaltsov rally. She said he was convicted without any witness testimony and without a lawyer present despite his objections.
