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Tens of thousands are gathering across Venezuela to march in protest against President Nicolás Maduro.
Four people are said to have died in clashes before the protests, as riot police mobilise to guard demonstrators.
National Assembly president Juan Guaidó, who organised the opposition march, is urging the army to disobey the government.
The US government has signalled its support for Mr Guaidó and called for President Maduro to go.
Early images from Wednesday’s protests show some demonstrators facing off against National Guard security forces in the capital Caracas, and tear gas being fired.
Anti-government protestors have turned up in “many cities” across the country, newspaper Tal Cual reported Wednesday.
Other demonstrations are being held in support of Mr Maduro.
Why are people protesting?
Venezuela has been in economic freefall for a number of years under the leadership of President Maduro.
Hyperinflation and key shortages, including of food, has forced millions out of the country.
Wednesday’s protests were called by Mr Guaidó after the president, who has been in power since 2013, was sworn into office for another six years earlier this month.
President Maduro was re-elected in May 2018 in a vote boycotted by the opposition and condemned by many in the international community.
US Vice-President Mike Pence has signalled support for the opposition.
In an opinion piece for the Wall Street Journal, Mr Pence said the US “strongly supports the National Assembly and Mr Guaidó”.
“Nicolás Maduro has no legitimate claim to power,” he wrote. “Nicolás Maduro must go.”
Venezuelan Vice-President Delcy Rodriguez dismissed Mr Pence’s intervention, reportedly telling a news conference “Yankee go home”.
What does the opposition want?
Opposition protestors are backing an attempt by Mr Guaidó to replace Mr Maduro and serve as an interim president ahead of new elections.
The governing Socialist Party announced a rival protest to counter the opposition march – and supporters on Wednesday have gathered for both sides.
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Anti-government protests broke out in the wake of an alleged soldiers’ rebellion
On Tuesday, Venezuela’s Supreme Court on Tuesday asked the state prosecutor to investigate whether Mr Guaidó had committed a crime.
Wednesday’s demonstrations come just two days after 27 National Guard soldiers allegedly revolted against the government at a guard post in the capital, Caracas.
Government officials dismissed it as a cover to steal weapons, but Mr Guaidó has urged others to follow in revolt.
Addressing soldiers on Monday, Mr Guaidó promised amnesty for those who refused to serve the government.
“We’re not asking you to launch a coup d’état, we’re not asking you to shoot,” he said. “We’re asking you not to shoot at us.”
On Wednesday the Congress leader tweeted (in Spanish): “Venezuela is reborn today in the streets in search of freedom and democracy”
Overnight protestors in the southern city of Puerto Ordaz reportedly toppled a statue of late socialist leader Hugo Chavez – a symbolic gesture against Mr Maduro’s government.
A 16-year-old with gunshot injuries was reportedly among those killed overnight on Tuesday, rights group the Venezuelan Observatory of Social Conflict says.