Fellow oppositionists Communist leader Sergei Kalyakin and Labor Party leader Alexander Bukhvostov were given similar "administrative arrest" sentences, to be served at police departments rather than in jail.
Police detained Milinkevich earlier Thursday at the editorial office of an independent business weekly.
On Wednesday, the opposition organized a 3,000-strong rally timed to coincide with the 20th anniversary of the Chernobyl disaster, the world's worst nuclear accident, which affected much of the country. Protesters held Ukrainian and EU flags, and chanted "Long Live Belarus!" and "Freedom!" and expressed support for Milinkevich.
Presidential elections held March 19 in the former Soviet republic that saw Alexander Lukashenko reelected to a third term with a massive 83% of the vote were condemned as undemocratic by international monitors.
The opposition also said the poll was rigged, and pledged to continue protests.
