Venezuelans are heading into uncharted political waters with the young leader of a newly united and combative opposition claiming to hold the presidency.
It comes as socialist president Nicolas Maduro was digging in for a fight with the Trump administration.
Violence flared again on Wednesday during big anti-government demonstrations across Venezuela.
At least seven protesters were reported to have been killed in the escalating confrontation with Mr Maduro, who has been increasingly accused of undemocratic behaviour, by the United States and other nations in the region.
Juan Guaido, the new leader of the opposition-controlled National Assembly, turned up the heat by declaring himself interim president before a mass of demonstrators in Caracas.
He said it is the only way to end the Maduro "dictatorship" in Venezuela, which has seen millions flee in recent years to escape sky-high inflation and food shortages.
"We know that this will have consequences," Mr Guaido shouted to the cheering crowd, then slipped away to an unknown location amid speculation he would soon be arrested.
The 35-year-old has reignited the hopes of Venezuela’s often beleaguered opposition by taking a rebellious tack amid the country’s crushing economic crisis.
Raising his right hand in unison with tens of thousands of supporters, he took a symbolic oath to assume executive powers: "Today, January 23, 2019, I swear to formally assume the powers of the national executive as president in charge of Venezuela."
In a united and coordinated front, the US, Canada and another dozen mostly Latin American countries, including Brazil, Colombia and Argentina, quickly announced they supported Mr Guaido’s claim to the presidency.
US president Donald Trump promised to use the "full weight" of US economic and diplomatic power to push for the restoration of Venezuela’s democracy.
"The people of Venezuela have courageously spoken out against Maduro and his regime and demanded freedom and the rule of law," he said in a statement.
Mr Maduro fired back by breaking diplomatic relations with the US, the biggest trading partner for the oil-exporting country, and ordering American diplomats to get out of the country within 72 hours, an order Washington said it would ignore.
The socialist leader, who so far has been backed by the military, as well as the government-packed courts and a constituent assembly, recalled the long history of heavy-handed US interventions in Latin America during the Cold War as he asked his allies for support.
"Don’t trust the gringos," he thundered to a crowd of red-shirted supporters gathered at the presidential palace.
"They don’t have friends or loyalties. They only have interests, guts and the ambition to take Venezuela’s oil, gas and gold."
Attention will shift to Washington on Thursday, where diplomats at the Organisation of American States will hold an emergency meeting on the Venezuelan situation.
The debate promises to be charged, and the National Assembly’s newly picked diplomatic envoy will be lobbying to take Venezuela’s seat from Mr Maduro’s ambassador.
Meanwhile, many Venezuelans will be looking for Mr Guaido to re-emerge and provide guidance on the opposition’s next steps.
The armed forces’ top command, which has so far remained silent, is also expected to issue a statement, although nobody expects the general’s loyalties to Mr Maduro to have shifted.