Ales Belyatsky, head of the human rights group Vesna, was arrested Thursday, his apartment and office were raided and a computer and documents seized, said Anastasia Loiko, a Vesna activist, RIA Novosti reported.
Belyatsky, accused of evading taxes, faces a maximum seven-year sentence.
Buzek, a former prime minister of Poland, expressed "great concern" about the arrest and called Belyatsky and others "unjustly detained political prisoners."
"Dragging people off the streets takes us back to the dark Communist past that we do not want repeated ever again," Buzek said in a statement. "This is unacceptable on our continent in the 21st century.
"I urge the Belarusian authorities to release Ales Belyatsky and the other unjustly detained political prisoners. Such repressive actions will only bring people closer together, uniting them to the final goal -- that of freedom and liberty."
Hundreds of people were arrested in July in Belarus, a former Soviet Republic, for their participation in protests against Belarusian President Alexander Lukashenko's authoritarian rule. Some demonstrators were beaten, reports said.