New protests are taking place in Moscow and other Russian cities following Vladimir Putin’s victory in last weekend’s presidential election.
Almost 20,000 people lined one of central Moscow’s main avenues.
However, this was much lower than the turnouts that followed December’s parliamentary election, which the opposition said was rigged.
Similar allegations have surrounded the presidential vote, which saw Vladimir Putin secure a third term.
Foreign states have accepted Vladimir Putin’s election but observers said the poll had been skewed in his favor.
Some opposition leaders had played down expectations of a huge turn-out on Saturday, partly because their movement had failed to stop Vladimir Putin securing another term.
The Moscow protest took place on Novy Arbat, a vast avenue lined by 1960’s skyscrapers.
Dozens of police and military vehicles were stationed on nearby streets.
The city authorities had allowed a rally of up to 50,000 people.
However, the figure appeared much lower than this. The opposition said 25,000 people had attended. Agence France-Presse quoted police as putting the figure closer to 10,000.
On a cold but sunny Moscow day, demonstrators waved banners and wore white ribbons – the symbol of the protest movement.
Protest organizer Vladimir Ryzhkov told the crowd: “These authorities are illegitimate. The same people are in power, the same people who took away our right to choose, the same people who destroyed freedom of speech and political competition.
“We will continue to demand deep political reforms and new elections.”
Sergei Udaltsov, one of the protest organizers, called for a million-strong march to take place in Moscow in May, a week before Vladimir Putin’s inauguration.
Another opposition leader, former chess grandmaster Garry Kasparov, told the crowd: “This was not an election. This was a special operation from a thug who wanted to return to the Kremlin.”
Sergei Udaltsov was among about a dozen protesters who were later arrested by police, AFP news agency said.
It added that police detained a number of people at an unauthorized rally on Saturday in St Petersburg.
Last Monday, a day after the election, police arrested about 250 protesters in Moscow and another 300 in St. Petersburg.
Vladimir Putin was re-elected for six years, having served two previous terms as president between 2000 and 2008.
On Friday, US President Barack Obama called Vladimir Putin from Air Force One “to congratulate him on his recent victory”, a White House statement said.
Barack Obama said he looked forward to hosting Vladimir Putin at the G8 Summit in May at Camp David.