More than 1,000 protesters have marched through the Venezuelan capital Caracas to demand that next week's municipal elections be free and fair.
Demonstrators accuse the National Electoral Council of bias
They claim that the National Electoral Council (CNE), which is due to supervise the 7 August polls, is biased in favour of President Hugo Chavez.
Supporters of Mr Chavez attacked the demonstrators by hurling stones, bottles and firecrackers at them.
Critics want anti-fraud measures to be introduced ahead of the vote.
The marchers wanted to reach the CNE's offices to deliver a document calling for the resignation of the body's top official.
They are also demanding an audit of Venezuela's electoral register, and asking for international observers to monitor the vote.
But police did not allow them to reach the CNE's headquarters.
The rally had to stop one block away from the council at metal barricades reportedly set up by police and troops in riot gear to keep pro- and anti-Chavez demonstrators apart.
Anti-Chavez factions claim the leadership of the CNE is controlled by a pro-government majority.
But the council proclaims its independence and insists the elections will be fair and transparent.
This is believed to have been one of the largest opposition rallies in recent months.
The opposition has accused Mr Chavez of using fraud to win a referendum on his rule last year, but international observers endorsed the results.