U.S. Ambassador to Russia Michael McFaul has said it is "discouraging" to see the detention of several hundred people in central Moscow during a rally in support of persons convicted as part of the Bolotnaya Square clashes case.

"Discouraging to see hundreds of detentions of those trying to exercise their constitutional rights of freedom of speech and expression, McFaul said on his Twitter page.

On Wednesday, McFaul is due to leave his position of ambassador and return to the U.S., where he will teach at Stanford University.

Several hundred people were detained by police near Moscow's Manezhnaya Square on February 24 for staging an unauthorized protest.

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