Before dawn on Tuesday, Mr. Guaidó posted a video statement calling for citizens to rise up against Mr. Maduro’s government as part of a “final phase” in the campaign against the socialist leader. Standing alongside national guardsmen as the sun rose over La Carlota air base in Caracas, Mr. Guaidó said his “nonviolent struggle” was within the parameters of the country’s Constitution.

“We count on the people of Venezuela — today the armed forces are clearly on the people's side, on the side of the Constitution, they are loyal to the Constitution, loyal to the people of Venezuela, to their families, to the future and to progress,” he said. “It has been years of sacrifice, persecution and even years of fear.”

Leopoldo López, a prominent opposition politician who had been under house arrest, stood alongside him after being released by soldiers who support the opposition. Mr. López led widespread street protests against Mr. Maduro in 2014 and was sentenced to nearly 14 years of detention.

But just hours after Mr. Guaidó spoke, the rebellion had taken on a different tone. The demonstrations in Caracas turned violent, as military and security officials loyal to Mr. Maduro clashed in the streets with opposition supporters.