Ilya Ponomaryov, a leftist member of the Russian parliament, said Friday that he has been visiting police lockups all over Moscow, asking people detained in this week’s election protests what party or organization brought them onto the streets.

None, they all told him. I came on my own. I learned about it on the Web.

“The Internet has arrived,” he said. “Facebook has arrived. Vkontakte [a Russian social-media site] has arrived.”

And on Saturday, thanks to the Web, organizers are expecting more than 30,000 people to demonstrate against what they see as the rigged results of last Sunday’s elections — because that’s how many have committed themselves to a sign-up sheet on Facebook. No political parties are recruiting protesters. No neighborhood committees are going around with checklists.

“This is not a political meeting,” Ponomaryov said. “This is a civil action.”

This week saw the first spontaneous political protests in Moscow in a long time. Via cellphone cameras and YouTube, people saw what they believed to be evidence of ballot-box rigging, and took to the streets in anger. If all those 30,000-plus people appear, Saturday’s demonstration— which has been almost ignored on television here — would be Russia’s biggest political demonstration since the early 1990s.

Other protests are scheduled in dozens of cities across Russia.

The Moscow gathering was originally planned for Revolution Square, across from the Bolshoi Theater, and the city permit that was issued limited it to 300 people. But when word went around that the square would be closed for repair work, the outrage flowed through Twitter like a torrent. The city backed down, but Thursday night it made an offer.

If the demonstrators would be willing to gather on Bolotnaya Square, across the river from the Kremlin, the city would raise the permitted number to 30,000. (By that time the Facebook list had already surpassed that figure.) Anastasia Udaltsova, whose husband is in jail after being detained on his way to a protest this week, said Friday that she and other organizers initially opposed the shift but then realized the value of holding such a big protest legally.

The question was how to notify the public. Television wasn’t likely to be much help. By Friday, however, the news was all over the Web.

Organizers are still planning to start the march in Revolution Square, in a nod to those who remain unwired, and proceed to Bolotnaya via a roundabout route that avoids Red Square, said Yevgenia Chirikova, another organizer, adding that the event is expected to be peaceful.

“These have been the most civilized protests in the world,” she said, referring to this week’s demonstrations, “with no broken windows and no upturned cars. Tomorrow will be an epochal day in the history of Russia.”

The organizers — a group formed under the auspices of Solidarity, a liberal democratic movement — have specific demands. They include new elections, the dismissal of the election commission and the immediate freeing of political prisoners. About 1,000 are in detention from this week’s arrests, they said.

Prime Minister Vladimir Putin is the focus of their anger. He solidified his hold on Russia, beginning 10 years ago, through what he called the “vertical of power,” which included total control of TV networks. The Internet here has always been quite free, but it was a niche phenomenon until recently, and its users tended not to go looking for news. In a sense, it was a safety valve, and the authorities up to now have left it alone.

But now, 37 million Russian adults use the Internet every day, according to a survey by the Public Opinion Institute here. About 52 million use it at least some of the time. And 11 percent of all users are in Moscow.

Certain Twitter hashtags were disrupted on election day, but this week there has been robust tweeting at #Dec10, and its Russian equivalent, about Saturday’s planned protest. Solidarity has a special Web site devoted to the march, the social networks have pages set up by committed subscribers, and bloggers have been writing about it, as well.

By coincidence, the protests are taking place on Human Rights Day. In a statement referring to protests this year in North Africa, Europe and the United States, Navi Pillay, the United Nations high commissioner for human rights, said: “The message of this unexpected global awakening was carried in the first instance not by the satellites of major media conglomerates, or conferences, or other traditional means — although these all played a role — but by the dynamic and irrepressible surge of social media. . . . In 2011, human rights went viral.”

The Russian demonstrations will be a measure of the extent to which Internet-based anger can find a footing in the off-line world here, where actual power resides. The police presence will be heavy, and organizers said they fear provocateurs from the security forces.

Unlike the authorities, Ponomaryov said, “we support the laws of the Russian Federation.”

The government has other means of dissuasion at its disposal, though. On Friday, according to the RIA Novosti news agency, Russia’s chief health officer warned people not to go the demonstration. They might, he said, catch the flu or even SARS.