The arrest comes as Belarus is under Western sanctions for persecuting opposition political parties, RIA Novosti reported.
Andrei Gaidukov, 22, an activist with the Belarusian Christian Democratic Party, was detained in Vitebsk and taken to a KGB pretrial detention center in Minsk, said party coordinator Tatyana Severinets.
Officials also detained BCDP member Ilya Bogdanov, who was released after pledging not to leave the city.
Neither man is prominent in the party's foundation, Severinets said.
If convicted of high treason, the men could serve a possible 15 years in prison.
The European Union, which has targeted Belarus for sanctions almost continuously since 1996, has criticized the country's September elections in which all 110 seats in Parliament went to candidates sympathetic to President Alexander Lukashenko.
Belarusian authorities have been accused by the West of denying basic rights and freedoms and failing to respect democratic principles.
The EU has placed travel bans and asset freezes on 240 Belarusian officials, including Lukashenko.