Alexander Milinkevich, who lost to incumbent President Alexander Lukashenko, said the rally would be peaceful and that there would be no clashes with police.
March 25 is popularly known as Freedom Day, an unofficial holiday marking Belarus' declaration of independence in 1918 and a traditional day for opposition demonstrations.
Early Friday morning Belarusian police forcibly broke up a five-day protest in central Minsk by opponents of Lukashenko and, according to Milinkevich, arrested about 500 people.
"According to our information, about 500 people were arrested," he said. "It is very difficult to obtain official information from the Interior Ministry."
He added that the protesters who had been arrested had been taken to various courts, though he did not know exactly which ones.
"We firmly demand that the authorities release the [detained] people who took part in a peaceful rally and ask the international community to support all those who suffered as a result of the pseudo-elections," Milinkevich said.
The Central Election Committee announced Thursday the final results of Sunday's elections, which were won by incumbent President Alexander Lukashenko with 83% of the vote, while Milinkevich garnered 6.1%. Election turnout was 92.9%, or more than 6.6 million people.
Opposition candidates denounced the elections as neither free nor fair, and called for a re-run. Thousands of demonstrators came onto the streets to protest Monday, but numbers dwindled in the face of freezing temperatures.