Updated

More than 20,000 protesters streamed down a central Moscow avenue Saturday to denounce Vladimir Putin's presidential election win, but the crowd's relatively small size compared to recent protests suggested the opposition movement has lost some momentum.

Some of the new political energy that has emerged in Russia in recent months, however, is being channeled into local politics and civic activism. Two men in their 20s who had both just won seats on municipal councils were among those who addressed the crowd Saturday to call on Muscovites to get involved in how their city is run, starting with their own neighborhoods.

The protest on Novy Arbat street ended peacefully, but leftist opposition leader Sergei Udaltsov and two of his followers were detained shortly afterward as they tried to march to nearby Pushkin Square.

Since the March 4 election, Udaltsov has returned to the confrontational approach of last year, when he spent a total of almost three months in jail as a result of defying city bans or restrictions on his protests.

Putin, who was Russia's president from 2000 to 2008 before switching to the prime minister's office due to term limits, won 64 percent of the vote. Because of changes in the length of the presidential term, he is set to return to the Kremlin for six years and would be eligible to run for six more.

His decision to return to the presidency infuriated opposition activists who have grown tired of his heavy-handed rule. A December parliamentary election that was manipulated to bolster Putin's party angered many ordinary Russians and bolstered opposition ranks.

Protests held after December's vote attracted up to 100,000 people in the largest show of discontent in Russia's post-Soviet history. On Saturday, the smaller crowd, surrounded by hundreds of troops and security forces, chanted: "We are the power!"

Although violations at the presidential election were numerous, observers viewed the vote as fairer than December's and said Putin's win was not in doubt.

But protesters said they do not recognize the results. "These weren't elections. This isn't a president," read a banner over the stage.

Opposition leader Garry Kasparov echoed that sentiment.

"This was not an election," Kasparov, a former world champion chess player, told the crowd.

"This was a special operation run by a thug who wanted to return to the Kremlin."

Russian actor Maksim Vitorgan, who was among thousands of people who volunteered to monitor the presidential vote, said "an amusement park would envy" the large-scale fraud he witnessed.

"We are all humiliated and insulted here," Vitorgan said. Putin "won the war for numbers. He's a president of numbers, not of the people."

"We know the truth, but what are we supposed to do with it?" Vitorgan added, voicing widespread concern that the opposition movement is losing its voice.

Other protesters, however, remained optimistic, even though Saturday's turnout could not match the tens of thousands who attended rallies in December and February.

Mikhail Solontsev, a 19-year-old student, who has taken part in opposition protests since December, said the pressure on Putin is already high. "It depends on us whether he will step down or not, but he's already scared of us," Solontsev said.

Police detained about 25 nationalists who broke away from the crowd at the start of the rally and tried to march. In St. Petersburg, police detained some 40 people out of several hundred who took part in an unsanctioned rally.

On Monday, the day after the election, Moscow police arrested some 250 people, including

Sergei Udaltsov, who had stayed on a central square after a protest rally had ended.