Police delivered Milinkevich, 58, to his home several hours before his scheduled release - apparently in an effort to prevent him from meeting with supporters who gathered outside the jail around the time they expected him to be freed.
Milinkevich returned to the jail later and spoke to about 100 supporters who stood outside, ignoring police warnings that their gathering was not sanctioned by authorities. "Even jail cannot deprive a truly free person of liberty," Milinkevich said.
Milinkevich, who ran unsuccessfully against authoritarian President Alexander Lukashenko in a March election, has been a unifying figure for an opposition that incorporates widely diverse forces ranging from democrats to Communists.
"We are fighting not against Lukashenko but for a new Belarus," Milinkevich said outside the jail.
The election, which the opposition and Western governments condemned as fraudulent, sparked unprecedented mass protests in this tightly controlled nation of 10 million. The protests in turn resulted in a wave of opposition detentions, with hundreds of activists jailed in ensuing weeks.
Lukashenko - often described by Western countries as "Europe's last dictator" - has been in power since 1994.
Also Friday, the Russian business newspaper Kommersant reported that Russian President Vladimir Putin had ordered an end to all subsidies to Belarus, a move that would have a serious effect on the country's economy and likely weaken Lukashenko.
About 75 percent of Belarus' economy is under state control, and the low prices that Russia has charged for oil and gas have been key to keeping the economy going. Belarus has recorded strong economic growth in recent years, which is widely seen as a key to support for Lukashenko.
The Kremlin press service said it was unaware of such an order, but the Belarusian opposition has suspected that Russia was on the verge of withdrawing some of its support. Already, Russian gas monopoly Gazprom has called on Belarus to triple the amount it pays for gas.
Deputy Ambassador of the Czech Republic to Russia, Lubos Vesely, was among 20 diplomats, who were expelled from the Russian Federation