Yan Dasgupta told Ekho Moskvy radio that Viktor Bout, 41, had been detained by Thai authorities solely to keep him in custody pending a formal extradition request from the United States.
"The Thai criminal case was opened in breach of the law... It is a serious crime, even by Thailand's own legal standards," the counsel said.
"It is simply outrageous - a complete violation of due process. We will take this matter up - there is no question about that," he added.
Bout was arrested in March in Bangkok during a joint police operation led by agents from the U.S. Drug Enforcement Administration (DEA).
DEA prosecutors claim that Bout conspired with others to sell millions of dollars worth of weapons to the Revolutionary Armed Forces of Colombia, a leftist group listed by the U.S. as a terrorist organization.
Dasgupta said however that no one in Thailand had ever intended to investigate either Bout's alleged crimes or put him on trial and that he had been detained because "the U.S. had failed to file a formal arrest warrant in time."
Earlier a Thai court postponed until July 28 the first hearing of Bout's extradition case as his Thai lawyer had developed "a heart problem."
Thailand received in early May a formal request from Washington to extradite Bout to the U.S., where he has been indicted on four charges: conspiracy to kill Americans and U.S. officers or employees, conspiring to provide material support to terrorists and conspiring to acquire and use an anti-aircraft missile.
Western law enforcement agencies consider Bout to be "the most prominent foreign businessman" involved in trafficking arms to UN-embargoed destinations.
UN reports say Bout set up a network of more than 50 cargo aircraft around the world to facilitate his arms shipments, earning the nickname 'the Merchant of Death.'