Tens of thousands of people filled a broad Moscow boulevard Saturday to demand a transformation of their country as protesters search for a way to keep up the pressure on Prime Minister Vladimir Putin.

The protests that have shaken the country in recent weeks have already won promises of major reforms. But as Russia’s long winter break closes in, it remains to be seen whether the excitement can be harnessed into a form that forces the new direction the diverse crowd is demanding.

Few protesters said they held out hope for rapid changes, and they will have to find a way to channel their still-vague frustrations into a movement that can be sustained for the long haul. The demonstrators have ranged from stylish young clubgoers to diminutive pensioners, all of whose lives were fundamentally transformed 20 years ago Sunday when the Soviet Union came to an end.

Now they are seeking another shake-up, as the torrent of social, economic and political forces that came after the hammer and sickle was lowered over the Kremlin for the last time has left the country traveling a current that is frustrating to many.

“We want to live in a free country,” said Timur Khutseev, 23, a theater aide who shivered in the freezing Moscow weather. “Our parents grew up under [Leonid] Brezhnev,” whose 18-year reign over the Soviet Union became a synonym for stagnation and repression. Putin, too, is seeking to extend his era to 18 years in March presidential elections. “We don’t want that,” Khutseev said.

The rally exceeded the size of one held two weeks ago, whose scale surprised even the organizers. On Saturday, they estimated, 120,000 people protested in temperatures that were in the teens. The Interior Ministry put the number at 29,000.

The challenge for organizers will be keeping up the fight. The movement’s strengths and weaknesses were on display Saturday, as many of the young, middle-class people who have been the driving force behind the sudden show of discontent this month said they remained cautious about politics in general even as they thought the country needed to change.

The protest comes shortly before a 10-day national holiday that includes New Year’s Day and Orthodox Christmas on Jan. 7, virtually shutting down the country. Organizers called for another protest in early February, and the March 4 presidential elections will help maintain focus, but if Putin is reelected and few changes follow, activists will need to find other ways to keep the crowds motivated.

“We don’t know who the leader might be, because there is no person who represents us,” Viktor Shenderovich, a popular writer, told the crowd. “But this is an expression of moral attitude. People don’t want to be stepped on.”

Two decades after he resigned from office, the last leader of the Soviet Union, Mikhail Gorbachev, told Putin to follow his example. “I would advise Vladimir Vladimirovich to go right now,” he told Ekho Moskvy radio, citing his own resignation on peaceful terms. “That’s what he should do too.”  But Gorbachev remains more influential  outside Russia than at home, and his opinion was unlikely to sway minds here.

Winning allies, promises

In just weeks, the protesters have won important victories, with President Dmitry Medvedev unveiling a sweeping package of reforms Thursday that would loosen the Kremlin’s tight grip on public life. On Saturday, the demonstration gained a surprising advocate, as Alexei Kudrin, a former finance minister and longtime member of Putin’s inner circle, appeared onstage to call for new parliamentary elections, one of the protesters’ key demands. The Dec. 4 vote garnered allegations of widespread fraud

“Elections should be held under new conditions,” Kudrin told the crowd, saying the current parliament should be allowed to work for the next six months or year but then be replaced. Without new elections, he said, “there will be a revolution.” He offered to serve as a mediator.

Another erstwhile Kremlin ally, billionaire Mikhail Prokhorov, who has said that he would run for president against Putin, was also in the crowd, though he did not speak.

Thousands packed Moscow’s Sakharov Prospekt, a street named for Soviet dissident Andrei Sakharov that is a mixture of boxy Communist-era buildings and glass-fronted ones built with the gusher of oil money that accompanied Putin’s ascent. First-time protesters exchanged nervous glances as police helicopters looped overhead and hundreds of police officers stood nearby.

But many people described rapid changes within their social circles: Before the parliamentary elections, they studiously avoided the simplest mention of politics in their private lives. On Saturday, with all their friends joining in the protest, many said they had little excuse not to muster the courage to come as well.

“Before, I didn’t think a lot about politics,” said Alexandra Anisimova, 29, a lawyer. “I’m still indifferent to any political party. But it’s my obligation as a citizen to be here.”

‘We should move forward’

Putin was initially dismissive of the protesters, suggesting earlier this month that they had been paid by foreign backers and that the white ribbons they have adopted as a symbol looked like condoms. But the Kremlin has since appeared to view the demonstrators as a serious threat, with even people who once enthusiastically supported Putin drifting into the other camp.

“Russia needed Putin at a certain stage, because he is a strong politician,” said Roman Chernykh, 30, who owns a chain of clothing stores. “But we should move forward. Today’s authorities are not doing anything for businessmen.”

Protesters exulted at the turnout and warned the government to expect more.

“I can see enough people to conquer the Kremlin and the White House right now. But we are not going to do it because we are peaceful,” said Alexei Navalny, a popular blogger who has emerged as a leader of the opposition, drawing roars from the crowd as he spoke from the stage. “But we cannot be patient forever.”