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Russian Prime Minister Vladimir Putin has accused the United States of being behind protests over the results of Russia's parliamentary elections.

Mr Putin said US Secretary of State Hillary Clinton "set the tone for some opposition activists".

She "gave them a signal, they heard this signal and started active work", he said.

Mrs Clinton maintained that her concerns were "well-founded". Election monitors have also been critical.

About 1,000 people have been arrested in Moscow during three days of protests alleging election fraud.

Organisers have called another protest for Saturday.

'Mercenary interests'

Mr Putin accused the protesters of acting "in accordance with a well-known scenario and in their own mercenary political interests".

He warned that those working for foreign governments to influence Russian politics would be held to account.

"It is unacceptable when foreign money is pumped into election processes," Mr Putin said in comments shown on state-run TV.

"We should think of forms of defence of our sovereignty, defence from interference from abroad," he added.

Mrs Clinton said the US supported the "rights and aspirations of the Russian people".

"We expressed concerns that we thought were well-founded about the conduct of the elections," Mrs Clinton told a news conference in Brussels after talks between Nato allies and Russian Foreign Minister Sergei Lavrov.

Most Russians did not want the kind of political upheavals that had been seen in recent years in Ukraine and Kyrgyzstan, he said.

Mr Putin's remarks came a day after he officially registered his candidacy for the presidential elections next March.

He stood down from the office in 2008 after serving his constitutional maximum of two consecutive terms, and has since held the post of prime minister.

'Serious concerns'

While maintaining that protesters had the right to express their opinion, Mr Putin warned that "if somebody breaks the law, then the authorities... should demand that the law is adhered to".

Monitors from the Organisation for Security and Co-operation in Europe (OSCE) said on Monday that there had been "severe problems with the counting process" after the vote, citing apparent irregularities such as the stuffing of ballot boxes.

Earlier this week the US expressed "serious concerns" over the conduct of the vote.

Russia's only independent election monitoring group, Golos - which is funded by the US and the EU - logged 5,300 allegations of electoral violations.

Its website was hacked and the head of the organisation detained for several hours on Sunday. Prosecutors fined Golos 30,000 rubles (£600; $958) for violations of the electoral law.

On Tuesday Russian Foreign Minister Sergei Lavrov responded angrily to comments Mrs Clinton made about the conduct of the elections during an OSCE meeting in Lithuania.

"This is not Hyde Park, this is not Triumfalnaya [Triumphal] Square in Moscow, where speakers arrive to pour out their soul and then turn around and leave, not listening to others," he said, according to Reuters.

Results published by Russia's Electoral Commission showed support for Mr Putin's United Russia party had dropped but that it would still retain a slim majority in the Duma.

Russian President Dmitry Medvedev insisted that the vote had been free and fair.