© Agence France-Presse/Kirill Kudryavtsev Riot police detain a Russian opposition activist taking part in an unauthorized rally, on Triumfalnaya Square in central Moscow.
Russian riot police arrested hundreds of activists in central Moscow on Tuesday to stop a new protest alleging that elections were rigged in favour of Vladimir Putin's ruling party.

Helmeted police in green camouflage and interior ministry troops deployed in force for an event that was organised through the Internet after a rare thousands-strong protest on Monday startled the authorities.

Opposition supporters shouted "Shame on you fascists!" and "Russia without Putin" in a tense stand-off with hundreds of pro-Kremlin youth who descended on the site in advance.

Those detained in Moscow included former cabinet minister turned Kremlin critic Boris Nemtsov -- released after being booked at a police station -- and several other members of opposition groups that were barred from running in the polls.

A Moscow police spokesman said 250 people had been thrown into waiting vans and taken to various police stations about two hours into the event. Interfax cited a security source as saying 300 had been detained.

Police in Putin's native city of Saint Petersburg also reported making around 200 arrests. The private Dozhd (Rain) television station said 50 towns and cities had seen protests in all -- most of them in the central Volga region.

Riot police on Monday had also detained 300 people who included prominent activist Ilya Yashin and opposition blogger Alexei Navalny. A court found both guilty on Tuesday of disobeying police orders and jailed them for 15 days.

The consecutive rallies came after Sunday's parliamentary election was won by Putin's United Russia party with a sharply reduced majority in a vote that the opposition said the authorities rigged to keep control of parliament.

International observers have reported major violations and said the State Duma poll was slanted in favour of United Russia.
© Agence France-Presse/Olga MaltsevaRiot police detain an opposition activist during an unauthorized rally, in Saint-Petersburg.

US Secretary of State Hillary Clinton initially expressed serious concern on Monday and reaffirmed that message during a Tuesday visit to the former Soviet republic of Lithuania.

"As we have seen in many places, and most recently in the Duma elections in Russia, elections that are neither free not fair have the same effect," Clinton said in Lithuania.

The Russian foreign ministry described Clinton's comments as "unacceptable" while President Dmitry Medvedev said it was "none of their business" what Russia's political system looked like.

Hundreds of opposition supporters -- many of them young and most responding to a call on the Internet -- tried to attend Tuesday's rally in Moscow.

But they were vastly outnumbered by pro-Kremlin youth who noisily beat drums to show their support of United Russia and chanted slogans such as "the future is with us".

Memorial human rights centre head Oleg Orlov told AFP at the rally that "police are behaving in a rough, rude manner, pushing people."

Interior ministry spokesman Oleg Yelnikov told AFP that security forces were now on a "heightened regime" of alert under which some 11,500 interior ministry troops were ensuring order nationwide.

Russian bloggers posted videos throughout the day Tuesday claiming they saw columns of armoured vans carrying troops heading down the main avenues into the city.

Meanwhile Putin played down the sharp election losses as an inevitable outcome of tough economic times. "There were losses, but they were inevitable ... Under the present circumstances, this is a good result," he said.

"We are not simply adapting, we are responding to the challenges of time, always and in everything. And yours truly too, I hope," he said in a televised meeting with supporters.

The opposition and many ordinary Russians have taken to calling United Russia the party of "swindlers and thieves" after a phrase first coined by Navalny.

Putin told his supporters not to pay attention because the authorities always had to struggle with "cliches".

The ruling party polled just under 50 percent of the vote after winning more than 64 percent in 2007. It lost 77 seats took just 238 places in the 450-members Duma.

United Russia's biggest opposition will be the Communist Party with 92 seats.