Alyaksandr Milinkevich was sentenced just hours after being arrested.
The same court today gave another two opposition figures -- Popular Front leader Vintsuk Vyachorka and Labour Party chairman Alyaksandr Buchvostau -- similar jail sentences for taking part in the rally.
A fourth opposition leader, Syarhey Kalyakin of the Communist Party, received a 14-day jail term.
The opposition called the march to protest against the government's handling of the consequences of Chornobyl disaster, the world's worst nuclear accident. The event, held on the 20th anniversary of the disaster, attracted thousands of demonstrators.
Opposition parties are reportedly considering staging a new march on May 1.
The opposition has staged several street protests since President Alyaksandr Lukashenka was reelected on March 19 in an election international organizations say failed to meet democratic standards.