Hundreds of protesters gathered to chant "Freedom!"
Demonstrators have taken to the streets of the Venezuelan capital Caracas to oppose moves to bar nearly 300 politicians from standing for election.
The supreme court has upheld the ban imposed by anti-corruption officials though no candidates concerned have been convicted of any crime.
Protesters say the ban reflects a further concentration of power in the hands of President Hugo Chavez.
Most of the candidates represent opposition parties.
There are less than four months to go now until regional and local elections which could prove a key test for President Chavez and his socialist revolution, the BBC's James Ingham reports from Caracas.
The mayor of one district in Caracas, now disqualified from standing for the post of city mayor, described the court's decision as a cowardly move.
"This decision was taken against millions of Venezuelan people, people who wanted and want change, and who will not give up on change," Leopoldo Lopez, mayor of Chacao, told protesters.
"This decision wants to stop Venezuelans from having hope in change."
The Venezuelan opposition, historically weak and divided, believes it can now make significant gains, our correspondent says.
But now some of its most high-profile politicians will be unable to compete because they have been blacklisted by the government, which accuses them of various misdemeanours in public office.
A delegation from the human-rights commission of the South American trading group Mercosur is in Venezuela investigating the opposition's claims.
But there is little that Mr Lopez and others can now do that will allow them to take part in November's polls, our correspondent says.
The government says it is serious about cracking down on corruption, arguing that the court's decision proves it is guaranteeing democracy.