CARACAS — Venezuela’s authoritarian leader, Nicolás Maduro, moved Sunday to consolidate his grip on power by taking control of the country’s last independent institution and sidelining the man who had staked a rival claim to the presidency.

In a chaotic legislative session in which security forces surrounded the National Assembly building, supporters of Mr. Maduro blocked the re-election of Juan Guaidó, the opposition leader, as the body’s head in an attempt to deprive him of the position that allowed him to challenge President Maduro’s leadership.

The Maduro administration’s move plunges a turbulent political situation further into chaos at a time when Venezuela is already facing an economic collapse and widespread hunger, which has forced millions of its citizens to flee.

Venezuela now has two men who assert they are the rightful president and two rival legislatures — one of which has two competing leaders, since Mr. Maduro’s supporters on Sunday elected one man to lead it, and Mr. Guaidó’s supporters gathered at a newspaper’s headquarters to re-elect him.

By seizing control of the National Assembly, Mr. Maduro, who also retains the backing of the security forces, would consolidate his control over the country.

But the move was immediately denounced by members of the Venezuelan opposition, who called it a “parliamentary coup d’état” and said there had been no quorum to call the vote.

Mr. Guaidó positioned himself as head of a caretaker government a year ago, just two weeks after being elected head of the assembly. Standing in the streets of Caracas with hundreds of thousands of Venezuelans, Mr. Guaidó said then that Mr. Maduro’s 2018 election was fraudulent. Invoking an article of the Constitution that transfers power to the head of the assembly if the presidency becomes vacant, he claimed the country’s leadership.

That announcement last year was quickly recognized by dozens of foreign governments, including the United States, which backed Mr. Guaidó by imposing crippling sanctions on Mr. Maduro’s government.

To continue to claim the interim presidency, Mr. Guaidó needed to be re-elected as head of the assembly on Sunday, according to analysts in and outside the country.

His victory had been expected, as the opposition controls the legislative body, but at the last minute, members of the National Guard prevented Mr. Guaidó and other supporters from entering the assembly’s white-walled building. Video footage showed Mr. Guaidó attempting to climb over the spiked metal fence to access the building where the vote would be held.

Inside, a member of Mr. Maduro’s party, Héctor Agüero, swore in the legislator Luis Parra as head of the assembly. There was no vote count.

Mr. Parra is a former member of the opposition who turned against Mr. Guaidó after Mr. Guaidó opened a corruption claim against him.

The United States immediately condemned the move.

On Twitter, Michael Kozak, acting assistant secretary for the State Department’s Bureau of Western Hemisphere Affairs, said that Mr. Guaidó “remains Venezuela’s interim president under its constitution,” adding that Sunday’s “phony National Assembly session lacked a legal quorum.”

In recent weeks, the opposition and the United States government accused Mr. Maduro of trying to block Mr. Guaidó’s re-election by bribing and harassing lawmakers. In December, Elliott Abrams, the United States special envoy to Venezuela, accused Mr. Maduro of trying to pay deputies up to $500,000 each to vote against Mr. Guaidó’s re-election.

In an interview just days after Christmas, Mr. Guaidó, 36, insisted he had the votes for re-election. But he acknowledged that Mr. Maduro was unpredictable and that anything could happen. “There is no manual,” he said, “for battling a dictatorship.”

But Sunday’s maneuver highlighted Mr. Guaidó’s embattled position: A year after he seemed poised to oust Mr. Maduro and bring relief to his once-wealthy nation, now facing economic collapse, he appears to be losing ground.

Mr. Guaidó coalition is in disarray, with dozens of his allies in exile, others in jail, and still others turning against him. Mr. Maduro’s grip on power is firm. And Venezuela’s humanitarian crisis is deepening, with millions in poverty, food shortages widespread and its people continuing to flee.

Mr. Guaidó’s own assessment of the past year is that he underestimated the government’s “ability to inflict harm” — meaning its willingness to threaten those who dissent. He cited the recent disappearance of one member of his party, Gilber Caro, who has not been seen since Dec. 20. “The persecution is brutal,” he said.