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, claiming 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.
The military has so far supported Mr Maduro and refused to back Mr Guaidó.
However the opposition leader has backing from dozens of countries, including most of Latin America and the United States.
A spokeswoman for the Spanish government warned against “bloodshed” in Venezuela. “Spain is not supporting any military coup,” Isabel Celaa said, urging a “peaceful” outcome to the Venezuelan crisis.
Colombian President Iván Duque called on the Venezuelan army to back Mr Guaidó.
White House press secretary Sarah Sanders said the US president had been briefed and “we are monitoring the ongoing situation”. US Senator Marco Rubio, a staunch supporter of Mr Guaidó, also took to Twitter to urge the military to give their backing to the National Assembly president.
In the video, Mr Guaidó says he has 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 says.
The video, published on Mr Guaidó’s Twitter account, shows him standing next to another opposition leader, Leopoldo López, who has been under house arrest after he was found guilty of inciting violence during anti-government protests in 2014.
Mr López said that he had been freed by members of the military who had declared their loyalty to Mr Guaidó.
The Venezuelan information minister, Jorge Rodríguez, responded to the events on Twitter, writing that the government was confronting a small group of “military traitors” which according to him were promoting a coup.
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 have been widely disputed.
The video appeared to have been recorded at dawn in or near La Carlota air force base in the capital Caracas.
Footage recorded later by Reuters news agency shows Mr Guaidó and Mr López with dozens of uniformed men on a highway in Caracas.
Many are wearing blue armbands and bandannas to signal their support for Mr Guaidó. The footage shows teargas being fired at the them.
Mr López, who leads the Popular Will party of which Mr Guaidó is a member, urged Venezuelans to join them: “All Venezuelans who want freedom should come here, disrupt order, join and encourage our soldiers, join our people. Good morning Venezuela, let’s do this together.”
Footage showed supporters of Mr Guaidó throwing stones at the airbase while others waved Venezuelan flags but their actions seemed to lack co-ordination.
Analysis by 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.
Venezuelan Defence Minister Vladimir Padrino said that military bases were “operating normally” and that the armed forces were “standing firm in defence of the constitution and its legitimate authorities”.
In a later tweet, he wrote: “They’re cowards!!”
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 Mr Maduro from “the right-wing conspiracy”.
“Every one of their actions will have an overwhelming response” he said. “We will be radical in our defence of the Bolivarian revolution,” he said.