Hundreds of people demanded an end to Vladimir Putin's 12-year rule in protests in Russia's two largest cities on Tuesday, despite a crackdown by riot police and spoiling tactics by pro-Kremlin youth.

Police said they had detained more than 400 people in St. Petersburg and Moscow as the protesters, emboldened by the prime minister's biggest election setback since he took power, tried to stage mass protests for the second successive day.

But police prevented many joining the Moscow rally and hundreds of pro-Putin youths in blue anoraks tried to spoil it, shouting "The people! Putin" to drown out their chants of "Russia without Putin" and "We want free elections!"

Boris Nemtsov, a liberal opposition leader, told Reuters he had been detained. Two opposition figures, blogger Alexei Navalny and Ilya Yashin, were jailed for 15 days for their role in Monday's opposition protest, the biggest in Moscow for years.

Despite his sentence, and the fall in attendance at Tuesday's rally in Moscow, Yashin said: "We are not going to stop our struggle."

The protests are a further sign of pressure on Putin to make changes after his United Russia lost ground in a parliamentary election on Sunday, even though European monitors and the opposition say it was slanted in the ruling party's favour.

The election outcome highlighted growing unease with the 59-year-old leader as he plans to reclaim the presidency next year and pointed to a mood shift after years of political domination by the former KGB spy who has brooked little dissent and marginalised 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 in Moscow.

"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 violent path as Libya and Syria.

"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.

US Republican Senator John McCain said Russia could now face a revolt. In a message to Putin on Twitter, he wrote: "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, 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 angered by widespread reports that the ruling party's vote count was inflated by ballot-box stuffing and voting irregularities.

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.

Putin, who ceded the presidency to Medvedev in 2008 because of a constitutional ban on three successive presidential terms, was booed at a sports event last month.

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."

Monday's rally in Moscow attracted about 5,000 people—a large number in a country where many protests are not allowed and demonstrators are often quickly carted away by police.

Police said about 300 people were detained after Monday's protest, and more than 50,000 police and Interior Ministry troops were out in force on Tuesday. Navalny and Yashin were both jailed in Moscow as the latest protests were 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.

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."

The political uncertainty has affected Russians stocks and the rouble. The rouble-denominated MICEX index fell by 3.8 percent, the dollar-based RTS index dropped by 4.7 percent and the rouble was 1.3 percent down against the US dollar.

Secretary of State Hillary Clinton reiterated US suggestions that Sunday's election was neither free nor fair. Russia's Foreign Ministry hit back, calling Clinton's remarks and White House criticism of the election "unacceptable".