Belarusian state television said protest leader Maria Kolesnikova was detained while trying to cross the border into Ukraine in the early hours of Tuesday morning, though two of her allies managed to leave the country.
The opposition movement had said masked men detained Kolesnikova in central Minsk on Monday and drove her off in a van.
State television denied this and said Kolesnikova had been removed from a car attempting to cross the border on Tuesday morning.