Viktor Bout, 41, is a former lieutenant in the Russian military who quit the armed forces in 1991. He then allegedly transformed himself into an international arms dealer.
Nicknamed "the Merchant of Death," Bout was arrested in a five-star hotel in Bangkok in what appears to have been a joint sting operation, involving law enforcement agencies from Thailand and the U.S.
"A Russian citizen has been arrested at the request of U.S. authorities in Thailand. This is unacceptable. We will demand Bout's extradition to Russia," Bout's lawyer, Viktor Burobin, said in an interview with the Moscow radio station Ekho Moskvy on Friday.
Earlier on Friday, Russia's Foreign Ministry confirmed Bout's arrest and said the Russian Embassy in Thailand was "maintaining contact" with Thai authorities and the detained businessman.
Burobin said the defense would use all available legal means to ensure Bout's extradition to Russia, but admitted that in this situation the options were limited.
He also said that Bout, who has been his client since 2002, could be extradited to the United States.
The U.S Justice Department earlier said they would seek Bout's extradition following new charges against him and his accomplice, Andrew Smulian, involving a deal to sell weapons to the Revolutionary Armed Forces of Colombia (FARC), which is designated a terrorist organization in the U.S.
According to the U.S. Drug Enforcement Agency (DEA), Bout and Smulian agreed in a series of phone calls and e-mails to sell and deliver surface-to-air missiles, helicopters and armor-piercing rocket launchers to two DEA informants posing as FARC members.
Bout has reportedly been trafficking weapons to Central and West Africa since the early 1990s. U.N. reports say he set up a network of more than 50 cargo aircraft around the world to facilitate his arms shipments.
He is considered by Western law enforcement agencies to be "the most prominent foreign businessman" involved in trafficking arms to UN-embargoed destinations from Bulgaria, Slovakia, Ukraine, Kyrgyzstan and other countries. Recent reports suggest he has also been operating in Iraq via front companies.
The Russian branch of Interpol confirmed on Thursday that both Belgium and Interpol had issued warrants for his arrest in 2002.
U.S. authorities, who suspect Bout of supplying weapons to Taliban and al-Qaeda militants, took measures against Bout in 2005, freezing his bank accounts and submitting a list of 30 companies linked to Bout to the UN Security Council Sanctions Committee.
Bout had earlier categorically denied accusations in an interview with the Russian media, saying "I have never supplied anything to or had contacts with the Taliban or al-Qaeda."
His lawyer also said on Friday that the Russian businessman had never been involved in criminal activities in Thailand, the U.S., or Russia.
"Russia's law enforcement bodies do not have anything against him [Bout]," Burobin said, referring to information received from Russia's Prosecutor General's Office.
He said a member of the defense team would soon fly to Thailand to study the materials making up the criminal case against Bout.
"Only after that we will be able to develop our legal position, but at this point, we must find out what he is being accused of," the lawyer said.