Speaking before he was charged at the Investigative Committee (SK), a federal police agency modelled on the FBI, he said he had committed no crime.
 
"Putin regime on trial here," he wrote on Twitter. "Come to SK to back me & other political prisoners!"
 
Two other activists are already in custody over the same allegations.
It was not immediately clear whether Mr Udaltsov, who is already subject to a travel ban, would also be arrested.
 
The plot allegation was first aired in a documentary on Russia's NTV channel, which the SK cited as evidence.
 
Activists are accused of seeking funds from Georgia, with which Russia fought a brief war in 2008, to stoke unrest in Russia.
 
The arrest of one of them, Leonid Razvozzhayev, who was apparently snatched off the street in the Ukrainian capital Kiev last Friday before being brought to Moscow, has caused concern in the US and outrage among human rights groups.
 
Mr Udaltsov, who leads the Left Front coalition, is a familiar figure from street protests in Moscow, for which he has served short periods in detention in the past.




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