Instructions: In this task, you are given an article. Your task is to summarize the article in a sentence.
Input: Mr Udaltsov is charged with organising "mass disorder" during a protest in Moscow in May 2012. He was arrested in October before being released.
The leader of the Left Front coalition has rejected the accusation, saying it is an attempt to discredit the opposition to President Vladimir Putin.
If convicted, Mr Udaltsov faces between four and 10 years in prison.
It is the first time that a Russian opposition activist has been placed under house arrest, the BBC's Oleg Boldyrev reports from Moscow.
Two other activists - Konstantin Lebedev, an aide to Mr Udaltsov, and Leonid Razvozzhayev, an aide to opposition MP Ilya Ponomarev - were charged alongside Mr Udaltsov in October but were remanded in custody.
Mr Razvozzhayev said he was abducted in Ukraine, smuggled into Russia and forced into signing a confession, which he later disavowed.
On Saturday, investigators asked that Mr Udaltsov be placed under house arrest by the court in Moscow's Basmanny district because he had not been exhibiting good behaviour and had not co-operated with them.
The court ruled that he stay inside his flat, limited his communications to his closest relatives and his lawyers, and banned him from using the internet, our correspondent reports.
The house arrest will last until early April when the investigation is expected to conclude.
Speaking to journalists before the verdict, Mr Udaltsov said he considered the new restrictions a purely political move, our correspondent says.
Some supporters in the courtroom shouted "shame on the judge" during the hearing, he adds.
The case against the three activists centres on a documentary broadcast by the pro-Kremlin television channel NTV which allegedly shows them discussing efforts to topple the Russian government with an official from neighbouring Georgia and seeking financial support.
Mr Udaltsov said the footage was a sham and the documentary "dirt and lies". He and the other two men pleaded not guilty in October.
In November, Investigative Committee spokesman Vladimir Markin said it had gathered new evidence supporting the allegation aired in the documentary that the activists had met foreign officials to plot mass riots.
He said their supporters had received special training abroad.
Analysts say Mr Udaltsov's arrest was part of a crackdown on the organisers of anti-government protests which began in December 2011 amid allegations that the parliamentary elections that month were rigged.
Output:
A court in Russia has placed the prominent opposition activist Sergei Udaltsov under house arrest.