Belarusian opposition leader Stanislav Shushkevich, a former chairman of the country's Supreme Council, said on Saturday he was detained on the Lithuanian-Belarusian border by Belarusian border guards.
"I was returning from Vilnius and was detained along with 12 students of the European Humanities University on the Belarusian territory after customs clearance," Shushkevich told RIA Novosti.
He said passports were taken from them.
Shushkevich heads the opposition Social Democratic Party.
The Belarusian border service has been unavailable for comment.
The detention comes ahead of a planned protest rally on Sunday, when Belarus celebrates its Independence Day.
Two groups, the Revolution through Social Networks and Strategy for the Future, are organizing new protests on Sunday against President Alexander Lukashenko's economic policies in the Belarusian capital Minsk and several other cities throughout the country.
Protests in Belarus are sometimes brutally dispersed by riot police, who also detain some journalists covering the events.
During the last protest, over 100 people were detained in the capital.
Lukashenko said earlier he would clamp down hard on anyone caught protesting against economic measures.
The Belarusian ruble has come under severe pressure in the first five months of the year from a large trade deficit, generous wage increases and loans granted by the government ahead of the December 2010 presidential elections, which spurred strong demand for foreign currency.
In the spring, the country's authorities devaluated the national currency by 36 percent, froze prices on some staple foods and introduced fuel rationing to keep the lid on the deepening crisis.