It was the boldest move yet by Juan Guaidó, Venezuela’s opposition leader: at sunrise, he stood flanked by soldiers at an air force base in the heart of the capital, saying rebellion was at hand.

Mutinous security forces supporting him had released the country’s most famed political prisoner. National guardsmen stood at Mr. Guaidó’s side as others joined marches to support the protesters that they had spent years repressing.

In the streets, anti-government demonstrators clashed with forces loyal to the president amid reports of live fire, rubber bullets and tear gas. A health clinic in Caracas took in 69 people injured during the day. An armored vehicle rammed protesters, but it was not immediately clear how many people were hurt.

Yet at the end of the day, Mr. Guaidó fell short of the prize he sought: the toppling of President Nicolás Maduro, who has relied on force, intimidation and widely discredited elections to remain in power.