Moscow police have warned they will prevent any unsanctioned protests in the Russian capital after the opposition promised to maintain demonstrations against elections won by Vladimir Putin's party.
"Any provocations will be prevented in strict accordance with the law," a Moscow police spokesman said in a statement to Russian news agencies.
"Those who try to stage any unsanctioned event understand perfectly well that they will be detained."
Police were holding several Russian opposition figures today after yesterday's protests.
Several thousand people took to the streets in central Moscow late yesterday, despite pouring rain, for a rally against the results of Sunday's elections in which Mr Putin's United Russia party won but with a sharply reduced majority.
Police said they arrested 300 people including prominent activist Ilya Yashin and opposition blogger Alexei Navalny after the protesters marched towards the Lubyanka Square that houses the FSB security service.
Around 250 people were still being held this morning, said Olga Shorina, a spokeswoman for the Solidarnost (Solidarity) movement that organised the protest.
Many now face 15 days of arrest, she said.
During the rally, called mostly through social networks, protesters chanted "Russia without Putin" and "Putin should be in prison."
Mr Navalny has won a huge following on the internet for exposing corruption at state-owned firms and he coined the phrase, which has now been taken up by all the opposition, "swindlers and thieves" to describe United Russia.
Mr Putin suffered his worst ever setback at the ballot box on Sunday as United Russia's majority in the State Duma was sharply reduced.
The opposition claimed the results would have been even more dramatic in clean elections.
Monitors led by the Organisation for Security and Cooperation in Europe said the polls were slanted in favour of United Russia and marred by "frequent procedural violations" including ballot stuffing. US Secretary of State Hillary Clinton raised "serious concerns".
Along with United Russia, three other opposition parties won seats in the State Duma, including the Communists.
However liberal party Yabloko failed to win sufficient votes for seats and another anti-Kremlin force, the Parnas party, was not even registered for the vote.
According to police, 2,000 people showed up at the Moscow rally while Shorina said up to 10,000 attended the rally and 1,500-2,000 marched later towards the offices of the Central Election Commission.
It was the largest protest in many years and a boost for Russia's embattled opposition, which traditionally struggles to mobilise protests in a country that has lost its taste for street politics in the turbulent 90s.
