Russians took to the streets of Moscow for the second successive day on Tuesday to demand an end to Prime Minister Vladimir Putin's 12-year rule, but riot police blocked their way and hundreds of pro-Kremlin youths tried to spoil the protest.

Ignoring warnings of a police crackdown, about 500 protesters chanted Russia without Putin! and Freedom! after opposition leaders used social media such as Twitter and Facebook to swell their ranks, witnesses said.

But hundreds more pro-Putin youths in blue anoraks also turned up at the protest in central Moscow and tried to drown out their chants by shouting: Russia, Putin!.

The crowd was held back by dozens of riot police and it appeared that opposition supporters were struggling to make it past police to the rally. Police said more than 100 people were detained as scuffles broke out.

Boris Nemtsov, a liberal opposition leader, told Reuters he had been detained and was being held at a police station.

The protests are the latest sign of pressure on Putin to make changes after his United Russia party lost ground in a parliamentary election on Sunday which showed growing unease with the 59-year-old leader as he plans to reclaim the presidency next year.

The vote pointed to a mood shift after years of political domination by the former KGB spy who has built up the image of a tough leader, partly by crushing a separatist rebellion in the Chechnya region but also by brooking little dissent and marginalising the liberal opposition.

We want fair elections. Look at what they have done to our country, our Russia, said a man who gave his name only as Alexei as he was detained by riot police.

We pay your wages, he screamed as they threw him into a bus.

But Maxim Mishenko, 34, said he did not want Russia to follow the same path as Libya and Syria into violence.

I don't want the same thing to happen here as in Libya or Syria. There will be no Slavic spring here in Russia if I have anything to do with it, he said.

U.S. Republican Senator John McCain wrote on Twitter that he believed Russia could now face a revolt, writing: Dear Vlad, The Arab Spring is coming to a neighborhood near you.

But many Russian political experts have dismissed suggestions that Putin could face an uprising in a country which has little tradition of major street protests, despite the 1917 Bolshevik Revolution, and where dissent has often been crushed.

PUTIN PROMISES CHANGES - BUT NOT NOW

Putin, meeting United Russia representatives, said he would reshuffle the government after the presidential election he is contesting next March but promised no immediate action.

It was not enough to appease opposition leaders emboldened by the decline in support for United Russia and angered by widespread reports that the ruling party's vote count was inflated by ballot-stuffing.

Even so, it won only a slim majority in the State Duma lower house. United Russia is set to have 238 of the 450 seats in the State Duma, 77 fewer than the 315 seats it won in 2007.

Putin's popularity ratings, although still high, have fallen this year and he upset many Russians by saying he planned to swap jobs with President Dmitry Medvedev after the presidential election, opening the way for him rule until 2024.

He was booed at a sports event last month and some voters fear his return to the presidency would herald a new era of economic and political stagnation in the world's biggest energy producer 20 years after the collapse of the Soviet Union.

It's clear that on the whole the legitimacy of the authorities is on the decline, Mikhail Prokhorov, a metals tycoon who fell out with the Kremlin after a brief political career, wrote in his blog.

If nothing changes, the whole (political) structure could collapse. This system will not last five years more.

BIGGEST OPPOSITION PROTEST IN YEARS

Monday's rally in Moscow attracted up to about 5,000 people and was widely described as the biggest opposition protest in the capital in years -- most are not allowed or the protesters are quickly carted away by police.

Police said about 300 people were detained after Monday's protest and a Moscow court sentenced Ilya Yashin, one of the organisers, to 15 days in jail. Another opposition figure, prominent blogger Alexei Navalny, also received a 15-day sentence as Tuesday's rally got under way.

This is no doubt a political decision aimed at intimidating me and my colleagues, Yashin said of his verdict, which he said could cause more discontent. We are not going to stop our struggle.

Putin's spokesman, Dmitry Peskov, said sanctioned rallies could continue to go ahead but added: The actions of those who hold unsanctioned demonstrations must be stopped in the appropriate way.

Secretary of State Hillary Clinton reiterated U.S. suggestions that Sunday's election was neither free nor fair and European monitors also said the election had been slanted in United Russia's favour.

Russia's Foreign Ministry called Clinton's remarks and White House criticism of the election unacceptable.

(Additional reporting by Gleb Bryanski and Alexei Anishchuk, Writing by Timothy Heritage, Editing by Steve Gutterman)