After tolerating the biggest protests of his 12-year rule while seeking election, Putin has signalled a harsher approach to dissent since the start of his new term as president.
In power since 2000, Putin easily won a six-year term on March 4 after four years serving as prime minister. His mantra of ensuring stability finds deep support among the elderly and many outside the cities, as have his strong measures against the protesters, accused by some of his backers of being spoilt urbanites financed by foreign powers.
But opposition leaders say Putin's heavy-handed tactics show that the former KGB spy is deeply worried by the protests that have undermined his once iron-clad authority.
On Friday, he signed a law increasing fines, in some cases more than 100-fold, for violations of public order at demonstrations, despite warnings from his human rights council that it was an unconstitutional infringement on free assembly.
Police and investigators raided the apartments of Udaltsov, anti-corruption blogger Alexei Navalny and socialite Ksenia Sobchak on Monday, seizing computer drives and discs, photographs and other belongings as armed guards stood outside.
"The authorities are in a panic," Udaltsov told reporters. "They are trying to conduct primitive, repressive actions, but I am sure they'll only achieve the opposite effect. These sorts of searches annoy and outrage people, and people in even greater numbers take to the streets."
Many protesters are middle-class city dwellers who have benefitted from the oil-fueled boom Russia has experienced during Putin's years in power but want more of a say in politics and fear his prolonged rule will bring economic stagnation.
They have turned to an opposition which is still in its infancy, lacks a clear leader and looks unlikely to topple Putin, still Russia's most popular politician, any time soon.
Mikhail, 34, an athlete from Moscow, barely contained his anger while he watched the march. "These people here are idiots. All those who think these protests can change something and bring something better than Putin to power are idiots," he said. "I don't know of anyone more adequate and better equipped to rule our nation and take it out of the crisis if necessary."