Belarusian opposition politician Maxim Znak was detained on Wednesday by masked men wearing plain clothes, the Interfax news agency cited his allies as saying.
Znak's reported detention followed the similar disappearance on Monday of Maria Kolesnikova, one of the leaders of mass protests against President Alexander Lukashenko that have rocked his government since a disputed Aug. 9 election.
Znak was the last member of the opposition's Coordination Council still active inside Belarus, other than Nobel prize-winning author Svetlana Alexievich, who has acted as a figurehead for the movement.
All the rest have fled or been forced abroad or been detained in a crackdown by Lukashenko's security forces as he seeks to maintain his 26-year grip on power in the former Soviet republic.
Znak's lawyer was cited as saying that his client's flat was also being searched by the state investigative committee and that he was subject to legal proceedings.
State investigators were also searching the headquarters of jailed opposition politician Viktor Babariko, a witness in Minsk said.
On Tuesday, prominent opposition leader Kolesnikova thwarted an attempt to deport her by tearing up her passport to avoid being forced to cross the border into Ukraine, two of her allies said.
She remains in detention.