Russia's economic reforms architect, Anatoly Chubais, has issued a statement questioning the arrest and imprisonment of opposition leader Boris Nemtsov.
Nemtsov and three other opposition figures all received jail terms of five to 15 days on Sunday in connection with a protest rally held Friday in central Moscow.
Chubais, who currently heads the Rosnano state technology firm, Monday rejected the court's reasons for the arrest, saying the credibility of the courts has been "undermined." He called on authorities to explain the detention.
The Reuters news agency says human rights leader Lev Ponomaryov bluntly called the arrest of Nemtsov and others "an open attack on the opposition."
Nemtsov was one of about 65 people arrested in the capital, while another 50 people were arrested at a similar rally in St. Petersburg.
Chubais insists the rally at which Nemtsov was detained was sanctioned, and said he does not believe the court-accepted justification for the arrest - that Nemtsov disobeyed a police officer.
Opposition leaders hold regular protests on the last day of each month with 31 days, to highlight the 31st article of the Russian constitution, which grants the right of assembly.
Moscow authorities had for months refused to sanction any opposition rallies in the city, prompting frequent police crackdowns and arrests when protesters ignored the ban.
That policy was revised several months ago to allow small, sanctioned demonstrations after President Dmitry Medvedev appointed a new Moscow mayor.