Police were deployed around the officially sanctioned site of the rally in central Minsk but did not intervene. However, about 10 activists who attempted to stage an improvised march across town were detained.
Lukashenka -- who has ruled Belarus since 1994 -- won another five-year term in a March vote that the opposition and Western officials denounced as fraudulent.
Demonstrators held signs reading: "Freedom to Political Prisoners!" and "Solidarity."
They urged authorities to free an opposition presidential candidate, Alyaksandr Kazulin, as well as jailed election observers and activists who campaigned against Lukashenka in the run-up to the presidential election.