Belarus' most prominent rights group said that its leader was detained Thursday in the capital and its office was searched by authorities.
Vladimir Labkovich, an activist with the Vesna rights group, said its leader Ales Belyatsky was detained by police outside his home in Minsk. Police and other officials refused to comment.
"We are extremely concerned about Belyatsky's safety and well-being," said Rachel Denber, deputy Europe and Central Asia director at Human Rights Watch. "Authorities are obviously targeting him because of his human rights work."
Vesna has been active in reporting on the police crackdown on a recent series of peaceful rallies that have reflected mounting public discontent against authoritarian President Alexander Lukashenko.
Lukashenko, who has ruled the nation of 10 million people for nearly 17 years, won another term in December's election that was criticized by international observers.
Vesna documented election violations and reported actively on the police crackdown on protesters on election night and subsequent repression of the opposition.
Lukashenko's government is facing a mounting financial crisis that has led to a sharp devaluation of the national currency.
In recent weeks, there have been a series of protests organized through social networks in which hundreds of people have marched down a central street of the Belarusian capital, clapping in unison. Police violently dispersed the rallies.