This was after Sunday's contested election to replace the late Hugo Chávez.
Police fired tear gas to disperse young demonstrators who threw rocks in an upscale district of Caracas after opposition presidential candidate Henrique Capriles refused to recognise the narrow victory of Chávez's preferred successor, Nicolas Maduro, and called for peaceful demonstrations.
"What happened yesterday was fraud, a lie. The opposition won and they know it," said Briand Alvar, who was among the protesters involved in the battle with police.
Capriles hopes protests will highlight the weakness of Maduro's mandate and stir up opposition outrage over accusations that the electoral authority is biased in favour of the ruling party.
Maduro, a 50-year-old former bus driver, called on his supporters to mobilise peacefully on Tuesday. He is due to be sworn in on Friday, likely leaving Capriles's call for demonstrations as a symbolic show of defiance.
Opposition sources told Reuters their count showed Capriles won by more than 300 000 votes. His team said it has evidence of some 3 200 election day irregularities, ranging from allegations of voters using fake IDs to intimidation of volunteers at polling centres.
Venezuela's electoral authority on Monday formally declared Maduro the winner of the election, saying he won 50.8% of the vote, against 49.0% for Capriles.
Threats and intimidation
"The majority is the majority. Democracy must be respected," said Maduro, who was handpicked by Chávez to continue his self-declared socialist revolution just weeks before dying of cancer on March 5. "The opposition cannot launch an ambush to jeopardise the will of the people."
In a sharply worded speech, the head of the National Electoral Council shot down Capriles's call for a recount and excoriated him for being disrespectful of Venezuelan law and institutions.
"Threats and intimidation are not the path to appeal the decisions of the National Electoral Council," said Tibisay Lucena, who opposition critics call a stooge of the ruling Socialist Party.
She also accused the US government and the Organisation of American States of trying to interfere in Venezuelan affairs after they backed the idea of a recount.
Opposition sympathisers flooded Twitter with the hashtag #CaprilesWonTibisayLied, which in the afternoon was the micro-blogging site's third most popular topic worldwide.
The official results showed Maduro winning by 265 000 votes but Capriles said he is sure he won and that he will only concede defeat if there is a full recount.
"All we're asking is that our rights and the will of the people be respected, and that every single vote be counted, every little piece of paper. That paper isn't for recycling, it's proof," Capriles said, urging his supporters to keep protests peaceful.
A new uncertainty
"We went out yesterday to vote against violence," he said. "We can never put ourselves on the side of violence."
The controversy around Venezuela's first presidential election without Chávez on the ballot in two decades ushered in new uncertainty in the Opec nation of 29-million people.
It also raised doubts about the future of "Chavismo," Chávez's self-proclaimed socialist movement, without its charismatic founder.
In his last public speech, Chávez named his long-time protégé Maduro as his preferred successor, giving him a huge boost heading into Sunday's election. But neither the endorsement nor the burst of sympathy following Chávez's death were enough to ensure an easy victory.
Maduro, who does not have Chávez's charm, saw his poll lead shrink in the final days of the campaign. Even then, the vote was a lot closer than most people expected.
Maduro's slim victory raises the possibility that he could face challenges from rivals within the disparate coalition that united around the towering figure of Chávez, who was an icon of the Latin American left.
Chávez comfortably beat Capriles by 11 percentage points and 1.6-million votes in October.
In this campaign, Capriles slammed Maduro as an incompetent and a poor copy of Chávez unable to fix the nation's many problems. He also offered a Brazilian-style mix of pro-business policies and strong welfare programmes.
Maduro was unable to match his former boss's electrifying speeches but nevertheless benefited from a well-oiled party machine and poor Venezuelans' fears that the opposition might abolish Chávez's slum development projects.