At least 16 people have reportedly been injured during street battles between police and supporters of Venezuela's President Hugo Chavez.
Venezuelans are deeply divided over President Chavez
Police used tear gas to disperse more than 100 supporters of the president, who were trying to disrupt an anti-government rally being held in their neighbourhood, a poor area of the capital Caracas.
The Chavez supporters responded by throwing bottles, stones and firebombs at the police, and destroying a police post.
The opposition had called for a demonstration in the eastern Petare neighbourhood - a Chavez stronghold - to prove that the president's support among the poor was fading.
Gunfire from unknown sources injured one police officer and three civilians, said Caracas fire chief Rodolfo Briceno.
At least 10 people were slightly hurt by flying objects, he said.
Two police officers were injured when they were accidentally hit by police vehicles, said Caracas Health Secretary Pedro Aristimuno.
The opposition protest took place despite an earlier appeal from the Interior and Justice Minister, General Lucas Rincon, to call the rally off.
A similar protest last month led to violent clashes in which one person was killed.
At least 50 people have been killed in street clashes and violence since April last year when Mr Chavez survived a short-lived coup.
In Friday's clashes, Chavez supporters burned tyres and blocked streets to prevent the opposition march.
The marchers refused to cancel the protest, only dispersing when the tear gas was used.
"I'm here for my grandchildren because I want a real democracy," opposition protester, Angelo Valles, 54, told the Associated Press news agency.
But a Chavez supporter, Rodolfo Garcia said: "While the opposition is looking for problems, Chavez is trying to help the poor."
Opposition leaders want to drive Mr Chavez out of office in a referendum expected after 19 August - halfway through his current term, which is due to end in early 2007.
The government has accepted to hold a referendum under a deal brokered by the Organization of American States in May.
The referendum has to be organised by a new National Electoral Council, but parliament has been deadlocked with government MPs staying away from proceedings.
A strike called in December by Chavez opponents struck a severe blow at the economy before finally petering out in January.