Moscow was uncommonly tense Wednesday, with tens of thousands of riot police patrolling the streets and helicopters buzzing overhead, while opposition leaders promised more flash-mob-type demonstrations to protest alleged official vote-rigging in last weekend's bitterly contested Duma elections.

For more than a decade, Russians appear to have quietly accepted Vladimir Putin's system of "managed democracy." The system utilizes a toolbox full of official measures to ensure that only Kremlin-approved parties and candidates get elected, and that the decisive share of votes is always won by the ruling party, United Russia (UR), which has been headed by Mr. Putin for much of its existence.

But on Monday, after official returns showed UR winning almost 50 percent of the votes – down sharply from the 64 percent it won in 2007 polls – up to 10,000 protesters, informed mainly through social media, converged on the downtown Chistye Prudhi metro station. They attempted to march to the Kremlin, shouting slogans like "down with the police state" and "Russia without Putin." About 300 were detained, and a few such as radical blogger Alexei Navalny and liberal opposition leader Ilya Yashin were subsequently handed 15-day prison sentences for "refusing to follow a lawful police order."

The next evening, hundreds more jostled with thousands of heavily-armored riot police on Moscow's downtown Triumph Square, and another 250 were detained, including former Deputy Prime Minister Boris Nemtsov, a co-leader of the banned liberal PARNAS party, and Sergei Mitrokhin, leader of the liberal Yabloko party, which officially won about 3 percent of the votes in Sunday's election. Protest rallies were also reported in other Russian cities Tuesday, including St. Petersburg, the Volga center of Samara, and the southern city of Rostov-on-Don.

"No one expected the public mood to snap like this; these rallies caught everyone by surprise," says Alexander Konovalov, president of the independent Institute for Strategic Assessments in Moscow.

"What is most remarkable is that the people we are seeing in the streets now are not the usual handful of hard-core protesters," who turn out for regular anti-Kremlin rallies on Triumph Square, he adds.

"These are completely new people, responsible, mature people, who are finally fed up with the open official lies and manipulations that everyone is expected to swallow, and see public protest as the only respectable option. Even a few weeks ago, for these people, taking to the streets would have been unthinkable. But now they feel pushed against the wall," he adds.

Opposition leaders say there will be more protests, including daily flash mobs and a big rally planned for Saturday in Revolution Square, which is adjacent to the Kremlin. That rally, planned weeks ago, has been granted an official permit – but only for a maximum of 300 participants, though organizers had asked to be allowed permission for 10,000 people – which the huge space could easily accommodate.

Most state media have not reported the anti-government protests, but have instead lavished coverage on the "Clean Victory" demonstrations that have been held each evening in downtown Moscow by members of the pro-Kremlin "Molodaya Gvardia" and "Nashi" youth groups. These organizations were created in the wake of Ukraine's Orange Revolution several years ago to play precisely such a counterbalancing role if similar disturbances were to occur in Russia.

"There is no revolution going on, just a few provocations," says Anton Smirnov, federal commissar of the Nashi movement. "We have had ten times more people at our meetings than the numbers of marginal people and paid fanatics," who come out to protest alleged election violations, he adds.

Not surprisingly, Russian social media such as Facebook, LiveJournal, and the Russian-language VKontakte have lit up with commentary, including first-hand witness accounts of official pressure and vote-rigging during the election, information about protest venues, and harrowing tales by arrested protesters of brutality at the hands of police.

One entry on the relatively new Openspace.ru, offers a wealth of helpful advice for first-time protesters, from what to bring with you, to how to behave at the rally, and how to get legal help when you need it: "If you are detained, do not resist, relax and press your chin to breast, cover your head with hands," it advises. "If you are beaten, don’t hesitate to shout, the louder the better.... Having found yourself inside the paddy wagon, immediately send a phone message. If you call, do it in secret, because they can seize your phone...."

Analysts say that the immediate response of the authorities, which has been to crack down hard, may be a symptom of weakness that is only likely to inflame the public mood.

"They say these protests are only happening in a few big cities, but that's where trends usually start," says Mikhail Vinogradov, chairman of Peterburskaya Politika, an independent St. Petersburg think-tank. "The reaction from authorities has been incoherent, and Plan A appears to be to nip these rallies in the bud through overwhelming police force. After that, they may try to make a few concessions. We'll see."

Former Prime Minister Mikhail Kasyanov, co-leader of the liberal PARNAS, which was banned from taking part in elections, says that Putin has virtually disappeared from public view as the protests have spread.

"Putin is not taking this as a lesson. He needs to move to engage with the opposition, seek dialogue and compromise, but he is not doing it," says Mr. Kasyanov, who was Putin's prime minister during his first term as president.

"What has happened this week is the beginning of the end for the Putin regime. Yes, he will probably be elected (in polls slated for March) but there will be more fraud, more protests, and public cynicism will grow. We can confidently predict that the lifespan of this regime will be no more than one to five years," he says.