Gunfire briefly crackled outside an air base in Caracas as the Venezuelan opposition leader urged the military to rise up against President Nicolas Maduro, saying there was no turning back.
Dozens of armed men in military uniform accompanied Juan Guaido and clashed with soldiers supporting Mr Maduro at a protest outside La Carlota air base.
Although the confrontation fizzled out, tens of thousands of people marched in Caracas in support of Mr Guaido on Tuesday, clashing with riot police along the main Francisco Fajardo thoroughfare.
A National Guard armoured car slammed into protesters who were throwing stones at the vehicle.
There were at least 36 people injured in Tuesday’s incidents, most of them hit with pellets or rubber bullets, said Dr Maggi Santi, of the Salud Chacao hospital in eastern Caracas.
Mr Guaido, in a video posted on Twitter earlier on Tuesday, said he had begun the "final phase" of his campaign to topple Mr Maduro, calling on Venezuelans and the military to back him.
Mr Guaido later left the rally he was holding with military supporters at the air base.
Mr Maduro said he had spoken with military leaders and that they had shown him "their total loyalty".
"Nerves of steel," he wrote on Twitter. "I call for maximum popular mobilisation to assure the victory of peace. We will win."
The move was Mr Guaido’s boldest effort yet to convince the military to rise up against Mr Maduro. If it fails, it could be seen as evidence that he lacks the support he says he has.
It might also encourage the authorities, who have already stripped him of parliamentary immunity and opened investigations into him, to make an arrest.
The US is among about 50 countries that recognise Mr Guaido as Venezuela's president, and has imposed sanctions to try to dislodge Mr Maduro, who they say won re-election last year through fraud.
A former US official said that while it was unclear whether Mr Guaido's efforts would spark a broader military uprising against Mr Maduro, they appeared aimed at making May Day street protests planned for Wednesday a turning point.
Mr Guaido has said the protests will be "the largest march in Venezuela's history" and part of what he calls the definitive phase of his attempt to take office and call elections.
Venezuela is mired in a deep economic crisis. Shortages of food and medicine have prompted more than three million citizens to emigrate in recent years.
Mr Guaido, leader of Venezuela's opposition-controlled National Assembly, in January invoked the constitution to take on the role of interim president, claiming Mr Maduro's re-election in 2018 was illegitimate.
US President Donald Trump weighed in on the power struggle late on Tuesday, threatening neighbouring Cuba with a full embargo if its troops he claimed supported the Maduro regime did not leave the country.
“If Cuban troops and militia do not immediately cease military and other operations for the purpose of causing death and destruction to the constitution of Venezuela, a full and complete embargo, together with highest-level sanctions, will be placed on the island of Cuba,” Mr Trump tweeted.
The Trump administration has accused Cuba of sending troops to support its ally Mr Maduro and block any attempts to overthrow him. The Cuban authorities have denied such claims and challenged the US to provide evidence of their presence.
Secretary of State Mike Pompeo told CNN that Mr Maduro was trying to leave the country to go to Havana, the Cuban capital, but was stopped by Russia.
Earlier on Tuesday, Mr Trump's National Security Adviser, John Bolton, said: “We want, as our principal objective, the peaceful transition of power.
"But I will say again as the president has said from the outset and as Nicolas Maduro and those supporting him, particularly those who are not Venezuelans, should know, all options are on the table."
Mr Bolton told the military in Caracas that it had a choice.
"Embrace democracy, protect civilians and members of the democratically elected National Assembly, or face more man-made suffering and isolation,” he said.
US envoy for Venezuela Elliott Abrams said there had been negotiations between people within the Maduro regime and outside the country to reach a deal and restore democracy.
“But it seems that today they are not going forward,” Mr Abrams said.
The US embassy in Venezuela issued a warning to its citizens to stay off the streets, while Republican Senator Rick Scott called for military involvement.
Mr Scott represents Florida, a state with a large Venezuelan community opposed to the Maduro regime, and called on Mr Trump to ready troops.
"The US military must be ready to supply humanitarian aid and defend freedom and democracy in Venezuela," he tweeted.
But there was no sign from the administration that it would send troops or increase its military involvement.
Brazilian leader Jair Bolsonaro said on Twitter on Tuesday that he supported a Guaido government and the "democratic transition" taking place in Venezuela.
"Brazil sympathises with the suffering Venezuelan people enslaved by a dictator," Mr Bolsonaro said. "We support the freedom of this sister nation to finally live in a true democracy."
But Turkish President Recep Tayyip Erdogan, after a call with Russian leader Vladimir Putin, renewed support for Mr Maduro and warned against a coup in the country.
Venezuelan Information Minister Jorge Rodriguez tweeted that the government was confronting a small group of "military traitors" trying to promote a coup.
Diosdado Cabello, head of the Constituent Assembly, a legislative body that acts in support of the government, said the opposition had not been able to take over the air base.
Mr Cabello urged Mr Maduro's backers to rally at the presidential palace in Caracas and support him.
Mr Guaido was accompanied by men in military uniform and opposition politician Leopoldo Lopez, who had been placed under house arrest.
"The national armed forces have taken the correct decision and they are counting on the support of the Venezuelan people," Mr Guaido said.
Mr Maduro appears to have kept control of state institutions and the loyalty of senior military officers.
He has called Mr Guaido a US-backed puppet who seeks to remove him in a coup.
The government has arrested Mr Guaido's top aide, stripped the opposition leader of his parliamentary immunity and opened several investigations into him.
It has also barred him from leaving the country, a ban he openly broke this year.
Last week, Mr Guaido said his congressional ally, opposition politician Gilber Caro, had been detained and that 11 members of his team had been summoned to appear before the Sebin intelligence agency.
Mr Lopez appeared to have left his home for the first time since being placed under house arrest in 2017, after three years in jail.
"I have been freed by soldiers on the side of the constitution and President Guaido," he tweeted. "All of us have to mobilise. It's time to win our freedom."
Rivas, a soldier in the group with Mr Guaido, denied government accusations that they had been tricked into backing the opposition leader.
"We're all afraid but we had to do it," Rivas said.
Spain, which is leading the EU on the matter, said that although it considered Mr Guaido to be the legitimate leader of Venezuela, it did not support a military coup, preferring elections.