Authorities in the Vitebsk region say Milinkevich and a local opposition activist were detained on suspicion that their car was involved in an earlier accident.
They later determined it was not the same car.
Milinkevich was in the region to gather signatures in support of candidates for local elections.
He ran against President Alyaksandr Lukashenka in March and led public protests after that disputed vote.
(Belapan, Interfax)