National Assembly head Juan Guaidó’s declaration follows demonstrations marking biggest show of opposition against Nicolás Maduro since 2017 crackdown.

The Trump administration recognized the newly appointed head of Venezuela’s congress as the interim president of the oil-rich nation, in a direct challenge to the nation’s authoritarian leader and his international backers. A White House tweet said President Trump “has officially recognized the President of the Venezuelan National Assembly, Juan Guaidó, as the Interim President of Venezuela.” The administration of President Nicolás Maduro didn’t immediately respond to the most serious challenge it has faced the six years it has governed this oil-rich nation. Moments before Mr. Guaidó spoke, the judges of Venezuela’s Supreme Court, which is controlled by Mr. Maduro, reiterated that any actions by the National Assembly would be considered null and void. On a sound stage on a street in Caracas, facing thousands of antigovernment demonstrators, Mr. Guaidó took the oath of office, holding up his right hand while holding the constitution in his left hand. He did so on a national holiday that is considered historic in Venezuela: Sixty-one years ago, Marcos Perez Jimenez became the last dictator forced from office. “Today, Jan. 23, 2019, in my capacity as president of the National Assembly, before God Almighty, Venezuela and my fellow deputies, I swear to formally assume the powers of the national executive as the president in charge of Venezuela,” Mr. Guaidó said. The Trump administration had in recent days signaled strong support for Mr. Guaidó, who on Jan. 5 had been named by fellow lawmakers to head the National Assembly. On Tuesday, Vice President Mike Pence declared in a video in which he mixed English with Spanish that the U.S. would support Mr. Guaidó. “We stand with you, and we will stay with you until democracy is restored,” Mr. Pence said.

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