MOSCOW (Reuters) - Vladimir Putin's opponents vowed on Sunday to press on with demonstrations against his 12-year domination of Russia after tens of thousands attended a march which kept up the momentum of their protest movement.
"We'll be back," the organizers said on a social network site, one day after demonstrators defied the cold to stage the third mass rally since anger mounted over alleged fraud in a parliamentary election won by Putin's party on December 4.
Waving flags and banners, protesters marched within sight of the red-brick Kremlin walls and towers on Saturday, chanting "Russia without Putin!" and "Give us back the elections!"
Although Putin's supporters also held a big rally in Moscow on Saturday, warning against the threat of instability, the size of the opposition march suggested Putin faces a protracted challenge as he prepares to return to the presidency in March 4.
Such protests were unthinkable six months ago and were sparked by demands for fair elections, but they have grown into one of the biggest political threats to the former KGB spy since he was first elected president in 2000.
Putin is all but certain to win a six-year term as head of state in March, but his authority has been damaged and a pause in protests over the long New Year holiday has not deterred people who protested in large numbers on December 10 and 24.
The longer the protests continue, the bigger the threat they pose to Putin's legitimacy, even though the opposition is only loosely united and contains groups as diverse as nationalists, leftists, liberals and non-affiliated environmentalists.
"When I saw the thermometer was at minus 22 C (-7.6 F) in the morning, I thought no more than 10,000-15,000 people would turn up. Thank God, I was very wrong. Muscovites turned out to be more determined, stronger and persistent than I thought," opposition politician Boris Nemtsov wrote in a blog.
"We face a protracted hard struggle against cynical, ruthless rogues and thieves. It's a marathon which we have to win," wrote Nemtsov, a cabinet minister in the 1990s, before Putin rose to power.
The next big protest is expected to be on February 26, the week before the presidential election, or soon after it, the organizers say.
Putin, 59, held up Saturday's pro-government rally as a sign that he still has support after eight years as president and four as prime minister, a position in which he has continued to be Russia's dominant leader since 2008.
Opinion polls show he remains Russia's most popular politician, and could win the presidential election by securing more than 50 percent of the votes in the first round.
Visiting a town on Saturday where he promised to move residents threatened by shifting ground on the edge of an open-pit coal mine, he said: "You see what we are doing. We deal with concrete problems of the people who live here."
But Putin's attempts to ignore opposition demands and then fob them off with token political reforms has not deterred them. Some protesters say they have been spurred on by the insults and jokes Putin has made at their expense.
Their demands range from a rerun of the December election and the dismissal of the central election commission chief to the registration of more political parties to the release of people they regard as political prisoners.
Frustration has grown since Putin and President Dmitry Medvedev, ushered into the Kremlin by Putin because the constitution barred him running for a third successive term in 2008, announced a plan on September 24 to swap jobs in the March election. The move was widely seen as flouting democracy.
"We have already reached a point of no return. People have stopped being afraid and see how strong they are together," said one protester, 49-year-old Ivan Kositsky.
Protesters had banners declaring: "Down with the cold, down with Putin." Other banners read "They froze our democracy" and "We are frozen in solidarity."
At the pro-Putin rally a few km across the capital, supporters of the prime minister said they feared the opposition protests would fuel instability - a charge aimed at them by Putin himself.
"We want everything to be good for pensioners, to be able to enjoy old age. We cannot live on the verge of a rebellion. We want nothing to change," said 66-year-old Larisa Orekhova.
The pro-Putin demonstrators carried posters saying "For Putin" with a check mark in a box next to his face. Another read "Putin led Russia out of civil war" and one said, "My children will live in Russia - I need Putin."
Additional reporting by Alissa de Carbonnel, Nastassia Astrasheuskaya, Thomas Grove, Alexei Kalmykov and Steve Gutterman