At least 250 anti-Putin protesters have been arrested in demonstrations Tuesday in Russia's capital, Moscow police told CNN.
The arrests came as demonstrators protested against the results of Sunday's parliamentary election.
Thousands of protesters had taken to the streets on Monday, and they hoped to mobilize such numbers again on Tuesday, but a much smaller turnout was witnessed.
Those who did come to protest were met with a very firm response by security forces, who dispersed many and made the arrests.
One observer of Russian politics said that the election mark the beginning of the downslide for Putin's influence, though he didn't think the prime minister's grip on power would fall immediately.
"Whatever else they reveal, Sunday's results undercut the image, common in the West, of Putin's regime as an effective authoritarian state. In fact, it is a regime that cannot even steal an election decisively," Daniel Treisman, professor of political science at the University of California, Los Angeles, and author of "The Return: Russia's Journey from Gorbachev to Medvedev," wrote in a column for CNN.
Earlier, a Russian opposition leader was arrested at an anti-government rally, he told CNN by phone.
Boris Nemtsov, a former deputy prime minister turned government opponent, was detained at a rally held to voice discontent over alleged fraud in the elections, which returned Prime Minister Vladimir Putin's party to power with a greatly diminished majority.
The opposition Other Russia party sent out invitations to the protest via Facebook under the title "Is the Revolution Continuing? Yes!"
About 800 people said they were coming to the rally, but protesters appeared to have largely dispersed as security forces moved into the area, a CNN correspondent at the scene said.
Putin's United Russia was holding a separate rally. Police said there were about 8,000 people there.
Russian security forces were on high alert pending the results of the elections, the Interior Ministry said. Officials did not say how many extra security forces were deployed or what units they were from.
Earlier, U.S. Secretary of State Hillary Clinton called for a "full investigation" of irregularities in the elections, citing "serious concern" about the vote.
Russia's Foreign Ministry dismissed the criticism Tuesday, saying "statements of U.S. Secretary of State Hillary Clinton concerning the parliamentary elections in Russia, as well as similar comments of the White House and State Department officials, are unacceptable."
Russia expects that "the U.S. will refrain from unfriendly invectives going against the common positive vector of development of our bilateral relations," the ministry said in an online statement.
Shortly after Clinton spoke, the Central Election Commission announced that Putin's party suffered a large loss of seats in the election.
United Russia will have 238 seats, down from more than 300 in the outgoing parliament, or Duma, Central Election Commission head Vladimir Churov announced, with 99.99% of ballots counted.
That will give it a slim majority in the 450-seat house.
United Russia polled just under 50%, preliminary results showed earlier. Meanwhile, the Communist Party will have 92 seats, A Just Russia will have 64, and the Liberal Democrats will have 56, Churov said.
Putin promised a reshuffle of the federal government and regional governors after the country's presidential elections, due in March, according to Russian news agency RIA Novosti.
Thousands of people protested against the election results Monday in Moscow and St. Petersburg, the state news agency RIA Novosti reported, calling the Moscow event the largest opposition demonstration in years.
People shouted "Putin is a thief!" and "Putin go away!" at the Moscow rally, which was attended by more than 5,000 people, photographer Irina Kruchkova said.
She said a "huge amount of people" had shown up, mostly middle-class people aged 25 to 40 who heard about the demonstration online.
About 300 people were arrested in Moscow, RIA Novosti reported, and about 120 people were detained in St. Petersburg.
Clinton was speaking in Vilnius, Lithuania, at a meeting of the Organization for Security and Cooperation in Europe, whose election monitors also criticized the voting.
The group was expected to discuss a draft report by the organization's election-observer mission. It details alleged attempts to stuff ballot boxes, manipulate voter lists and harass election monitors.
A preliminary OSCE report on the election said some political parties had been prevented from running and the vote was "slanted in favor of the ruling party."
The group, which monitors and promotes democracy and human rights in Europe, cited the lack of an independent body running the election or impartial news media.
And there was "undue interference of state authorities" in the vote, the 56-member OSCE said in a statement.
As Russian Foreign Minister Sergey Lavrov looked on, Clinton told the group the United States had "serious concern about the conduct of the elections," and called for an investigation of reported fraud and intimidation.
"The Russian people, like people everywhere, deserve the right to have their voices heard and their votes counted," Clinton said. "And that means they deserve free, fair, transparent elections and leaders who are accountable to them."