Tens of thousands of protesters gathered in Moscow on Saturday, some chanting "Russia without Putin," weeks after parliamentary elections that were marked by allegations of fraud. The rally was even larger than the protests held two weeks ago, Western news outlets reported, and included smaller rallies in cities and towns across Russia.

The protests showed, the Associated Press reported, that "the protest movement ignited by the fraud-tainted Dec. 4 parliamentary election may be growing."

Alexei Navalny, a corruption-fighting lawyer and popular blogger, electrified the crowd when he took the stage. A rousing speaker, he had protesters shouting "We are the power!"

Navalny spent 15 days in jail for leading a protest on Dec. 5 that unexpectedly drew more than 5,000 people and set off the chain of demonstrations. Since his release, he has helped to further galvanize the opposition.

Putin's United Russia party lost 25 percent of its seats in the election, but hung onto a majority in parliament through what independent observers said was widespread fraud. United Russia, seen as representing a corrupt bureaucracy, has become known as the party of crooks and thieves, a phrase coined by Navalny.

"We have enough people here to take the Kremlin," he shouted to the crowd. "But we are peaceful people and we won't do that — yet. But if these crooks and thieves keep cheating us, we will take what is ours."

Vladimir Putin will seek election to a new term as president in March, after an interval as prime minister in which he was widely viewed as the power behind President Dmitri Medvedev. The New York Times reports that the protests have "shaken" the Russian administration, which at first misjudged the depth and degree of popular outrage at the perceived rigging of the vote.

The protests have shaken the Kremlin, which has not encountered widespread public resistance since Mr. Putin became president in 1999. Mr. Putin initially sneered at the protesters, saying that the white ribbons they have adopted as a symbol resembled limp condoms, and that they only participated because they were paid by foreign agents seeking to undermine Russia.

But it has become clear that the Kremlin is taking the protesters’ complaints as a warning signal, and is willing to make concessions to head off a more dangerous confrontation. President Dmitri A. Medvedev on Thursday proposed a package of deep political reforms, and high-level Kremlin-connected figures on Saturday aligned themselves with the demonstrators for the first time.

Organizers, including Navalny, threatened that future demonstrations could bring out one million protesters or more.

The BBC reported similar rallies across the country. Several protesters were arrested in St. Petersburg, Putin's hometown, and between 800 and 1,500 people gathered in Novosibirsk, in Siberia, among other rallies.

Update: Mikhail Gorbachev, the former Soviet president, told a Moscow radio station Saturday that Putin should follow his example and resign, the Associated Press reported. Gorbachev's remarks followed an interview in a liberal newspaper earlier this week in which Gorbachev called Putin's response to protests "shameful" and "embarrassing."