Spanish police detained nine people in Madrid in mid-November, including Alexander Gofshtein, a lawyer for the bankrupt Russian oil company Yukos, on a warrant issued by Spain's Audiencia Nacional, or National High Court.
Genrikh Padva told RIA Novosti: "Russian lawyers consider the current situation unprecedented."
He said international principles regarding confidential meetings between client and counsel, and a lawyer's right to free movement and immunity were violated with Gofshtein's arrest.
Padva also said the arrest "demonstrated disrespect for Russia."
He said Russian defense lawyers have encountered problems finding a Spanish lawyer for Gofshtein, because Spanish lawyers "are afraid they could be detained, as well."
Padva said his client does not understand the reason for his detention, adding that the term of Gofshtein's preliminary custody is still unknown.
Gofshtein traveled from Moscow to Spain for a meeting in the Soto del Real prison with his client, Zakhar Kalashov, a purported Georgian mafia boss alleged to be involved in money laundering.
Spanish police said earlier in a press release that Gofshtein was arrested as part of a large-scale operation codenamed Operacion Avispa (Operation Hornet) targeting Russian and Georgian mafia members operating in the country.
They said he brought money to his jailed client, Kalashov, whose criminal organization made large sums from criminal operations in Moscow, including illegal gambling, kidnapping and extortion, and then invested them in Spanish real estate, restaurants and other assets.
Alexei Mitrofanov, a flamboyant lawmaker from the ultra-nationalist LDPR party, told RIA Novosti that a group of Russian parliamentarians could fly to Spain in late January to resolve the problem, which has affected bilateral relations.