By Lidia Kelly and Thomas Grove
MOSCOW (Reuters) - Thousands of Russians chanting "Time for change" challenged Vladimir Putin's presidential election victory on Saturday but their numbers were far fewer than in previous weeks.
People who demonstrated in bright sunshine in central Moscow waved flags, balloons and banners and wore white ribbons, the symbol of protests that began three months ago. Echoing chants from previous rallies, they shouted "Russia without Putin".
"The road will be long and hard, it will be no quick struggle, but we will do it all. Russia will be free - Russia demands change!" liberal leader Grigory Yavlinsky told the crowd.
But organizers put the crowd at 25,000, about a quarter of the size of the last protest before the March 4 poll gave Putin a six-year third term as president. Police estimated the crowd at 10,000 and independent witnesses put it at under 20,000.
Even though international vote monitors say the election was skewed in his favor, opposition leaders have been forced by the margin of victory to acknowledge that Putin was the winner.
Officials results showed the prime minister and former KGB spy won almost 64 percent of votes and put the runner-up, Communist Gennady Zyuganov, on less than 18 percent.
The opposition is struggling to find a way to maintain pressure on Putin and mount a sustained challenge to the man they say has stunted Russia's political and economic development after 12 years rule as president or prime minister.
Some are defiant. Sergei Udaltsov, a far-left leader, called for 1 million people to march on May 1, a national holiday.
"Only the street can change the authorities. Only the masses. We have no other option. That's why we'll be fighting, going onto the streets - until we overthrow them," he said.
But organizers did not agree on a date for the next protest and many said the opposition has to be patient over its demands for a more open political system and greater democracy.
"If this system took 15 or so years to be created, we need a few years - three, four, five - to dismantle it," Yavlinsky said.
Fewer protesters are optimistic about the chances of enacting change since police detained hundreds of people who attended unsanctioned protests on Monday in Moscow and St Petersburg or refused to go home after a Moscow rally that had been permitted.
Putin, 59, has made clear he will allow protests agreed ahead of time with city authorities but will crack down against rallies that are not sanctioned.
Helmeted riot police pulled Udaltsov from the top of a garbage bin and detained him, along with two others, after he tried to lead dozens of supporters to a central Moscow square after the sanctioned rally ended.
In St Petersburg, Russia's second city, police hauled away about 30 to 40 people when more than 200 staged an unsanctioned, mostly silent protest without banners, witnesses said. Police said 280 were detained in the city on Monday.
Police used batons to break up an unsanctioned protest in Russia's third biggest city, Nizhny Novgorod, and detained about 50 people, Interfax news agency said.
Some protesters remain defiant, demanding more rallies and protests to follow the wave that began over allegations of fraud in a parliamentary poll won by Putin's party on December 4.
"The 64 percent Putin says he received - he barely got above 50 percent if you take out all the violations. The presidential elections were a joke," Sergei Nemchinov, 50, who is unemployed, said at the Moscow rally.
"Everything depends on the price of oil. The authorities would already have fallen if it weren't for the price of oil and gas. If it does fall, Putin won't last another two years."
Others, deflated by the lack of success in drawing concessions out of Putin and worried about this week's detentions, showed their feelings by not attending.
Police were out in force and two helicopters hovered overhead as the rally got under way on the modern Novy Arbat street in central Moscow. Riot police were also out in force near Red Square in case the protesters tried to march.
"We need to create a modern society. That's why protests are needed. We cannot let lies win," said Dmitry Yakovlev, 27, a consultant.
He hoped arrests would be avoided: "But if it comes to it - I am ready to defend the truth, to suffer for truth."
(Additional reporting by Maria Tsvetkova; Writing by Timothy Heritage; Editing by Steve Gutterman and Sophie Hares)