Venezuela's President Nicholas Maduro vowed victory over the protesters, as the opposition leader called for a fresh round of protests.
There were clashes on the streets of the Venezuelan capital, spurred by opposition leader Juan Guaido's call on the military to rise up against President Nicolas Maduro, who said he had defeated an attempted coup.
A carefully planned attempt by Mr Guaido to demonstrate growing military support disintegrated into rioting as palls of black smoke rose over eastern Caracas.
Mr Maduro declared victory over the uprising, congratulating the armed forces for having "defeated this small group that intended to spread violence through putschist skirmishes."
"This will not go unpunished," Maduro said in an address broadcast on television and the radio.
"Prosecutors will launch criminal prosecutions for the serious crimes that have been committed against the constitution, the rule of law and the right to peace."
The opposition leader rallied his supporters with an early morning video message that showed him, for the first time, with armed troops he said took months of urging to join his campaign to oust Maduro.
The 35-year-old National Assembly leader was filmed outside the La Carlota air base, where he asked the armed forces inside to join him.
The video had the extra shock value of featuring key opposition figure Leopoldo Lopez at his side, saying soldiers had released him from years of house arrest.
Mr Guaido said the move was the "beginning of the end" of Maduro's regime, and there was "no turning back."
He later called for a fresh round of protests on Wednesday, in another video message
"I am calling on the armed forces to continue their march in 'Operation Freedom' tomorrow, " he said.
"We will be on the streets."
Venezuela's health services said earlier at least 69 people were injured, two from gunfire, during clashes between demonstrators and security forces.
It was unclear how much of the military supported him.
A senior Brazilian official said a number of Venezuelan troops have sought asylum in Brazil's embassy in Caracas, saying there had been "various applications" from Venezuelan soldiers.
He would not say how many or what ranks, but Brazilian media reported 25 had applied so far.
The applications came after Brazil's president Jair Bolsonaro threw his support behind Venezuelans "enslaved by a dictator," a reference to President Nicolas Maduro whom Mr Guaido is challenging for power.
Brazil is one among 50 countries supporting Mr Guaido.
Thousands of opposition supporters flocked onto a highway near the air base, many waving Venezuelan flags, but they were met with gunfire and tear-gas fired by soldiers at the compound's perimeter.
Soldiers backing Guaido wore blue armbands to demonstrate their allegiance to the opposition leader recognized as interim president by more than 50 countries, but there appeared to be few of them.
Several military vehicles ran into a crowd of protesters, injuring some of them.
CNN said it was taken off the air by the Venezuelan government moments after it broadcast the video.
There were reports that the BBC was also pulled off air.
A plume of black smoke rose from an area near a helicopter hangar on the base, where demonstrators who briefly managed to enter were pushed back.
"Today is the day Maduro resigns. Today is the day all the country's drug dealers resign. Today we have a Venezuela. Today we have a nation," said one protester amid the confusion.
Mr Guaido appeared alongside high-profile opposition politician Leopoldo Lopez who had been put under house arrest by Mr Maduro's regime, but who announced he had been "freed" by soldiers supporting Mr Guaido.
Mr Lopez later entered the Chilean embassy with his wife and one of his children to claim asylum, before moving to the Spanish embassy, Chile's Foreign Minister Roberto Ampuero announced in Santiago.
As UN Secretary General Antonio Guterres appealed to all sides to avoid violence, Venezuela's army chief and defense minister, General Vladimir Padrino, issued a stark warning of possible "bloodshed" - adding that he would hold the opposition responsible.
In a message on Twitter, Mr Padrino said the situation in military barracks and bases in the country was "normal."
He later said an army colonel had received a bullet wound to the neck during the clashes in Caracas.
Communications Minister Jorge Rodriguez called on the army "to remain on maximum alert to - with our glorious National Bolivarian Armed Forces - defeat the attempted coup and preserve peace."
US and EU support
The US, meanwhile, threw its full support behind Guaido, with the White House calling on the military to protect the people and support the country's "legitimate institutions," including the opposition-controlled National Assembly.
"The U.S. Government fully supports the Venezuelan people in their quest for freedom and democracy. Democracy cannot be defeated," US Secretary of State Mike Pompeo said on Twitter.
US Secretary of State Mike Pompeo said the US had "indications" that Mr Maduro was ready to leave the country, but that "Russia convinced him to stay".
Mr Maduro denied these claims, saying "Mister Pompeo, please, this really is a joke".
U.S. envoy for Venezuela Elliott Abrams said the protests in Caracas were part of a broader effort negotiated with three senior officials in President Nicolas Maduro's government to restore constitutional order.
Mr Abrams, speaking to reporters at the State Department, said the talks involved Venezuela's Defense Minister Vladimir Padrino, Supreme Court chief justice Maikel Moreno and presidential guard commander Ivan Rafael Hernandez Dela about restoring democracy in the country.
"There have been some interesting negotiations among Venezuelans inside the regime and outside the regime about returning to the constitution," Abrams said. "They negotiatied for a long time the means of restoring democracy but it seems that today they are not going forward," he added.
The European Union called for "utmost restraint" in the crisis.
"The EU is closely following the latest events in Venezuela. We reiterate that there can only be a political, peaceful and democratic way out for the multiple crises the country is facing," EU diplomatic chief Federica Mogherini said in a statement.
Ms Mogherini said the EU "firmly stands with the Venezuelan people and its legitimate democratic aspirations".
On the other hand, Russia, Mr Maduro's main backer and creditor with China, accused Mr Guaido of "fueling conflict" in the oil-rich country.
Mr Maduro's leftist Latin American allies Cuba and Bolivia also condemned Guaido.
President Ivan Duque of neighboring Colombia -- home to more than a million refugees from Mr Maduro's regime - called on Twitter for "soldiers and the people of Venezuela to place themselves on the right side of history, rejecting dictatorship and Mr Maduro's usurpation."
Internet observatory NetBlocks reported "multiple internet services" were restricted in Venezuela following Mr Guaido's appeal.
Spain warned against bloodshed and said it was "not supporting any military coup".
In his video, recorded outside the La Carlota base, Mr Guaido said the "definitive phase" had begun in his attempt to oust Mr Maduro - who has presided over a catastrophic economic implosion since taking over from his late mentor Hugo Chavez in 2013.
"Brave soldiers, brave patriots, brave men supporting the constitution have answered our call," Mr Guaido said.
Tensions in Venezuela have been ratcheted up to a critical level this year, after Mr Guaido, who is head of the opposition-ruled National Assembly, announced January 23 that he was the acting president under the constitution.
He said Mr Maduro had been fraudulently re-elected last year.
Yet although US President Donald Trump has repeatedly said "all options" are on the table regarding Venezuela - including, implicitly, military action - there has been no noticeable US military mobilization.
Instead, Washington has upped the economic pressure, through sanctions aimed at Mr Maduro's regime and by cutting sales of Venezuelan oil - the South American country's main revenue earner.
It also warned against any attempt to arrest Mr Guaido, who has been left free to roam Venezuela and hold rallies.
Mr Maduro and his government have repeatedly accused the United States of trying to foment a coup, and blame the economic devastation in the country on the tightening US sanctions.