Opposition leaders defied the Russian authorities on Tuesday by organising a second mass protest in two days against Vladimir Putin's 12-year rule, despite warnings of a police crackdown and the jailing of one of the organisers.

Using social media such as Twitter and Facebook, they summoned supporters to a rally in Moscow, one day after up to 5,000 people protested in the capital over alleged vote-rigging in Sunday's election and demanded an end to Putin's rule.

Reporters said about 600 people gathered for the new protest but they included many pro-Kremlin youth in blue anoraks who had also turned up, chanting: Russia, Putin! while the opposition protesters shouted Freedom! and Russia without 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. At least 10 people had been detained as scuffles broke out.

Under pressure to make changes after his United Russia lost ground in Sunday's parliamentary election, Putin said he would reshuffle the government after a 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.

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, was still in detention.

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.

WARNING OF ARAB-SPRING STYLE REVOLT

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.

This has little practical impact. United Russia can even muster the two-thirds majority needed to make constitutional changes, or even impeach the president, if it forges alliances with other parties.

But the vote points 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 has lost his aura of invincibility.

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.

Secretary of State Hillary Clinton reiterated U.S. suggestions that Sunday's election was neither free nor fair after the opposition complained that vote-rigging had inflated support for United Russia.

European monitors also said the election had been slanted in United Russia's favour. U.S. Republican Senator John McCain went further, warning on Twitter: Dear Vlad, The Arab Spring is coming to a neighbourhood near you.

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 dissent has often been crushed.

But 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 March 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.

Putin seemed to rule out immediate change by telling United Russia members he would make significant renewal of personnel in the government only after the March 4 presidential election.

Asked whether voters would see a new Putin, he replied that Russia had to modernise and reduce its dependence on energy exports, adding: In that sense we all change, myself included.

(Additional reporting by Guy Faulconbridge, Writing by Timothy Heritage, Editing by Steve Gutterman)