Witnesses reported that at least one person was hurt on Sunday when Chavez supporters allegedly threw stones, bottles and firecrackers at opposition supporters visiting a local mayor detained at security police headquarters.
It was the first violent incident since Chavez and his opponents began their formal campaigns leading up to the referendum on the leftist president's rule.
Armed officers guarding the Caracas headquarters of the DISIP political police did not intervene to stop the attack, witnesses also said.
Supporters of Henrique Capriles, the mayor of Caracas'
Baruta district who has been detained for alleged incitement
since 11 May, scattered when masked youths attacked them.
At least one of the alleged attackers carried a poster urging
voters to support Chavez in the referendum.
The violence occurred as Chavez was delivering a speech on his radio and television show Hello President.
Chavez showed what he said was a document proving that the US-based National Endowment for Democracy gave $300,000 to several opposition groups.
"The government of Mr George W Bush continues putting its
hands where it should not put them," said Chavez, who will face a recall referendum on 15 August.
The opposition gathered millions of signatures to challenge Chavez's rule in the polls.
Chavez said the money given by the endowment was used by the opposition to draft a government plan titled Country Consensus as a blueprint for a post-Chavez Venezuela.
The former paratrooper accused the opposition of "selling its
sole to the devil" to remove him from power.