Venezuelan authorities say they are putting down a small coup attempt after opposition leader Juan Guaidó announced he was in the "final phase" of ending President Nicolás Maduro's rule.
He appeared in a video with uniformed men, saying he had military support.
Mr Guaidó, 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 Guaidó'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, the armed forces 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.
What is the latest?
Protesters supporting both sides have gathered at different points in the capital, Caracas.
There are running clashes between Mr Guaidó'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.
Meanwhile, Venezuelan Defence Minister Vladimir Padrino 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.
He also revealed one soldier had suffered a bullet wound.
What sparked the protests?
A three-minute video by Mr Guaidó was published in the early hours of Tuesday. In it, he announced he had the support of "brave soldiers" in Caracas.
"The National Armed Forces have taken the correct decision... they are guaranteed to be on the right side of history," he said.
He was filmed alongside another opposition leader, Leopoldo López, who has been under house arrest since being found guilty of inciting violence during anti-government protests in 2014.
Mr López, who leads the Popular Will party of which Mr Guaidó 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 Guaidó, 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.
The most dramatic moment yet
Analysis by BBC diplomatic correspondent Jonathan Marcus
Events in Venezuela are uncertain and unclear but Mr Guaidó has seemingly gambled heavily in his latest bid for power.
The loyalty of the military to President Nicolás Maduro's regime has been the central factor that has kept him in power. Barring a fundamental change in their allegiance or at the very least a significant split in their ranks, no amount of outside diplomatic pressure is going to push Mr Maduro from office.
So are we seeing this split now? Mr Guaidó and his supporters claim parts of the military across the country have backed him, but so far there is little evidence of this.
For Mr Guaidó the stakes are huge. The Venezuelan government says it is putting down a coup attempt. This is perhaps the most dramatic moment yet in Venezuela's current political saga.
Read more about Venezuela's crisis:
How has the government reacted?
The Venezuelan Information Minister, Jorge Rodríguez, wrote on Twitter that the government was confronting a small group of "military traitors" who, according to him, were promoting a coup.
Mr Maduro, meanwhile, said he had spoken to the leaders of the armed forces who had "expressed their total loyalty to the People, the Constitution and the Fatherland".
A senior member of the governing socialist party, Diosdado Cabello, called on supporters of President Maduro to take to the streets around the presidential palace to defend him from "the right-wing conspiracy".
Foreign Minister Jorge Arreaza told Reuters news agency the "coup attempt" had been "directly planned in Washington, in the Pentagon and Department of State, and by [National Security Advisor John] Bolton".
What is the international community saying?
- UN Secretary General Antonio Guterres appealed for both sides to avoid violence
- US Secretary of State Mike Pompeo sent a tweet backing Mr Guaidó. "The U.S. Government fully supports the Venezuelan people in their quest for freedom and democracy," he wrote
- Mr Bolton warned Mr Maduro not to use force against civilians
- Spain warned against bloodshed and said was "not supporting any military coup".
- Brazilian President Jair Bolsonaro - who backs Mr Guaidó - has called an emergency meeting with his defence minister, foreign minister and vice-president.
- Colombian President Iván Duque called on the Venezuelan army to back Mr Guaidó
- Bolivia's President Evo Morales and Cuba's Foreign Minister Bruno Rodríguez have sent messages of support to their ally Mr Maduro, condemning what they called the "coup d'etat" in Venezuela.