The Belarusian security service said on Sunday it had detained prominent businessman Yury Chizh on suspicion of large-scale tax evasion.
Chizh, owner of Belarusian firm Triple, had been a close adviser to authoritarian Belarusian President Alexander Lukashenko since the 1990s but business sources say he fell out of favour in recent months. He now faces up to seven years in prison if convicted.
Spokespersons at Chizh's company, Triple, could not be reached for comment on Saturday and Sunday.
"Together with him (Chizh) there were detained a series of people," said Dmitry Pobyarzhin, a representative of the KGB security service. "They are all now in a KGB detention centre."
Opposition parties have called Chizh President Lukashenko's "wallet". In 2012 he was included in a list of EU sanctions imposed on Belarusian individuals and companies in protest at the country's human rights violations.
Belarusian media had reported on Friday that Chizh had been detained, but this was not confirmed at the time by the KGB.
The Belarusian government controls 70 percent of the former Soviet republic's economy, and the biggest companies are known for having close links with the authorities.
The EU ended five years of sanctions against Belarus and Lukashenko in February, citing improving human rights.
According to tax data, Triple made a net loss of 51.5 billion Belarusian roubles (around $2.5 million) in the first nine months of 2015.