As part of a new initiative, activists collected contact information and addresses from demonstrators to make it easier to organize civic actions on a neighborhood level.
Gleb Pavlovsky, a former Kremlin political consultant, who attended Saturday's rally, estimated that up to 500,000 people have taken part in the protests in Moscow, a city of 11.5 million.
He said the Kremlin has not figured out how to deal with the protest movement.
"Therefore, they alternate between taking tough action and stepping back from confrontation," Pavlovsky said. "For the Kremlin, it is very worrying that Moscow no longer supports Putin, but it is very important that this is purely a Moscow phenomenon."
Although opposition protests also were held Saturday in several other Russian cities, the largest, in St. Petersburg, drew only a few thousand people. Protests elsewhere attracted only hundreds or even dozens. About 100 attended an unsanctioned rally in Nizhny Novgorod and about 20 of them were detained.
The Moscow organizers had spent days in tense talks with the city government over the protest route for Saturday, typical of the bargaining that has preceded each of the opposition marches.
A protest on the eve of Putin's inauguration ended in clashes with police, and the Kremlin responded by arresting some of the participants and approving a new draconian law that raised fines 150-fold for taking part in unsanctioned protests. The city, however, granted permission for the subsequent opposition rally in June, which was peaceful.
A day before the weekend rally, parliament expelled an opposition lawmaker who had turned against the Kremlin and joined the protest movement. Anger over the ouster of Gennady Gudkov may have helped to swell the ranks of the protesters.
"Russia no longer has a constitution," Gudkov told the rally. "Russia no longer has rights, and Russia no longer has a parliament worthy of respect. Shame on this parliament, and shame on this government!"
Gudkov's expulsion also means he loses his immunity from prosecution, and his supporters fear he could face arrest.
His son, Dmitry Gudkov, also a lawmaker, said he hopes the Kremlin will think twice about arresting his father after seeing the size of the protest. "They will either have to think about serious reforms and end their repressions, or they will come to a very bad end," he said as marched with a column of protesters.
"It's necessary right now for all Russians to come out into the streets to show the regime that changes are needed in our country, and that without them our country can't develop," said teacher Valentina Merkulova, who participated in Saturday's protest. "The most important thing is that, the more Russians come out, the less bloody the change of regime, the change of power. A change of power is necessary."
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