Venezuelan authorities say they are putting down a small coup attempt after opposition leader Juan Guaido announced he was in the "final phase" of ending President Nicolas Maduro's rule.
He appeared in a video with uniformed men, saying he had military support.
Mr Guaido, who declared himself interim president in January, called for more members of the military to help him end Mr Maduro's "usurpation" of power.
But military leaders appeared to be standing behind Mr Maduro.
Venezuela's defence minister appeared on television to stress the point. However, photos from Caracas show some soldiers aligning themselves with Mr Guaido's supporters.
Mr Maduro's detractors hope the military will change its allegiance as resentment grows following years of hyperinflation, power cuts, food and medicine shortages.
So far, they have stood by Mr Maduro - despite dozens of countries, including the UK, the US and most of Latin America, recognising Mr Guaidó as Venezuela's rightful leader.
As a result, John Bolton, the US national security adviser, said what was taking place in Venezuela was not a coup, but a legitimate leader trying to take control
Protesters supporting both sides have gathered at different points in the capital, Caracas.
There are running clashes between Mr Guaido's supporters and armed military vehicles. Protesters were also seen throwing rocks, but being repelled by tear gas and water cannon.
Television cameras also caught the moment armoured vehicles drove into a crowd but it is unclear if there were any injuries.
El Universal newspaper said that at least 37 people had been injured across Caracas.
The BBC's Guillermo Olmo, in Caracas, said Tuesday marked the most violent episode of the Venezuelan political crisis so far.
Venezuelan Defence Minister Vladimir Padrino appeared on television to emphasise that the military remained behind Mr Maduro.
He said the uprising by some members of the military had been "partly defeated", but warned of possible bloodshed.
"The weapons of the republic are here to defend the nation's sovereignty and independence," he warned.
The protests came after a three-minute video by Mr Guaido was published in the early hours of Tuesday. In it, he announced he had the support of "brave soldiers" in Caracas.
"People of Venezuela, let's take to the streets... to support the end of the usurpation, which is irreversible... The National Armed Forces have taken the correct decision, they have the support of the people of Venezuela, and the backing of our constitution, they are guaranteed to be on the right side of history," he said.
He was filmed alongside another opposition leader, Leopoldo Lopez, who has been under house arrest since being found guilty of inciting violence during anti-government protests in 2014.
Mr Lopez, who leads the Popular Will party of which Mr Guaido is a member, said he had been freed by members of the military.
He went on to urge Venezuelans to join them on the streets.
But later in the day, Chile's Ministry of Foreign Affairs revealed he, his wife Lilian Tintori and their daughter had entered Chile's embassy in Caracas to seek protection.
Mr Guaido, the president of the opposition-controlled National Assembly, has been calling on the military to back him ever since he declared himself interim president.
He argues that President Maduro is a "usurper" because he was re-elected in polls that had been widely disputed.
The video appeared to have been recorded at dawn in or near La Carlota air force base in Caracas.