A spokesperson for opposition leader Alexaner Kozulin said a protester apparently injured during clashes with the police later died in a local hospital. Two more protesters have been missing since they accompanied Kozulin in an attempt to negotiate with the police, the spokesman said.
Official sources denied all knowledge of the incident.
Kozulin, one of three defeated candidates in the March 19 elections, was arrested during a rally on March 25 and has been kept in a pre-trial detention center 30 kilometers (about 19 miles) from Minsk. He has been charged with serious public order offenses and hooliganism, and could face up to five years in prison if convicted.
Incumbent leader Alexander Lukashenko was re-elected to a third presidential term March 19 with 83% of the vote. Opposition candidates - all of whom polled in single figures - denounced the elections as rigged, and staged a five-day protest on central Oktyabrskaya Square in Minsk until police disbanded them on March 24.
The next day, crowds protesting Lukashenko's re-election clashed with riot police, who detained 300 people.
Lukashenko has been dubbed "Europe's last dictator" by Washington, but has domestic support within the country for providing relative stability in comparison with some other former Soviet states.
Russia's Foreign Ministry earlier said that the March 25 events in Minsk were a failed attempt to repeat the tactics used by opposition groups in other former Soviet republics to protest election results.