Alexander Voloshin, Uralkali's well-connected chairman, denounced as "outrageous" and "simply absurd" the arrest by Belarusian authorities of Vladislav Baumgertner, the potash group's chief executive.
Mr Voloshin - once a senior aide to former Russian president Boris Yeltsin, who was rehired temporarily by current president Vladimir Putin – urged the "immediate release" of Mr Baumgertner, who was arrested in Minsk, the Belarusian capital, on Monday.
"I know Vladislav Baumgertner to be an honest and very decent law-abiding citizen, as well as a world-class professional," said Mr Voloshin, who is also on the board of Norilsk Nickel, the Russian metals giant, where he was chairman from 2008-10.
Allegations made against Mr Baumgertner by Belarusian officials, who have accused him of abusing his position as chairman of the Belarusian Potash Company cartel, "look highly irregular".
BPC power actually rests with the cartel's general manager, a Belarusian appointment, Mr Voloshin said.
"In light of the above, accusations of abuse of power against Vladislav Baumgertner look simply absurd."
'Strange and inadequate'
The comments represent an intensification in Russian reaction to Mr Baumgertner's arrest on charges of "abusive exercise of power and abuse of office", a charge which can reportedly carry a 10-year jail sentence in Belarus.
Earlier Uralkali released a statement saying that it "refutes any allegation of wrongdoing on the part of Mr Baumgertner or any other manager of Uralkali".
Belarus has issued warrants for the arrest of four other senior Uralkali staff, with RIA Novosti reporting that Belarus had also intended to arrest Mr Voloshin and Suleyman Kerimov, the top Uralkali shareholder, but these
Igor Shuvalov, the Russian first deputy prime minister, had termed the situation "strange and inadequate", the Interfax news agency said.
The arrest follows Uralkali's withdrawal from BPC late in July, a decision which Mr Baumgertner said reflected "unacceptable" actions by Belarus in permitting potash exports outside the consortium, which had controlled more than 40% of world trade.
"Such actions… have ultimately destroyed the fundamentals of our prolonged fruitful co-operation," Mr Baumgertner said at the time, forecasting that the break-up of BPC would lead to a collapse in potash prices from some $400 a tonne potentially below $300 a tonne as Uralkali relied on volumes to boost revenues.
For Belarus, which relies on potash for some 20% of its budget, lower values of the fertilizer are seen as representing a serious blow to the financial welfare of a country which in 2011 received a $3bn bailout from Russia.