The court found on October 3 that the arrest of the former world chess champion and eight other activists ahead of an antigovernment demonstration in Moscow had been "disproportionate to the aim of maintaining public order."
It also ruled that Russia had violated two articles of the European Convention on Human Rights that cover the right to a fair trial and freedom of assembly and association.
The applicants were convicted for having breached the regulations on holding demonstrations and were ordered to pay a fine.
The European court judgment is not final. Parties in the case have three months to appeal.