The move is the latest of several that undercut prospects of free elections next year, despite commitments made to the Biden administration in return for sanctions relief.Venezuela’s top prosecutor accused several top opposition figures of treason and ordered their arrest on Wednesday, the latest blow to prospects for credible elections that the government has agreed to hold next year in exchange for the lifting of crippling U.S. economic sanctions.The attorney general, Tarek William Saab, said that opponents of the leftist government had accepted money from ExxonMobil to sabotage President Nicolás Maduro’s recent referendum on annexing a large, oil-rich region in Guyana. The oil company could not immediately be reached for comment.Mr. Saab did not say what, specifically, the accused had done to thwart the referendum, but he said they would be charged with treason, conspiracy, money laundering and criminal association. He announced arrest warrants for 15 people, some of them prominent opposition members, including people who live abroad and two U.S. citizens.The Biden administration has tried to coax Venezuela into holding elections, relaxing some of the damaging American sanctions. In October, the government reached an agreement with the opposition on steps toward a vote, and it agreed last week that candidates who have been barred from running for office could appeal that penalty to the country’s top tribunal.But Mr. Maduro’s government has also repeatedly undercut the opposition’s ability to mount a meaningful challenge.More than 2.4 million Venezuelans voted in October in an opposition primary election for president, held without official government support. Since then, the government has questioned the primary’s legitimacy, has taken legal aim at its organizers and has barred the winner of the primary, María Corina Machado, from running for office for 15 years, claiming that she did not complete her declaration of assets and income when she was a legislator. Three of those Mr. Saab accused on Wednesday are members of Ms. Machado’s political party who live in Venezuela.Since Mr. Maduro took power in 2013, after the death of Hugo Chávez, the combination of growing oppression, rampant corruption and sanctions has made life much harder for ordinary Venezuelans, and millions have left the country. Under Mr. Maduro, international observers have called the country’s elections illegitimate.With the allegations of treason, President Biden must decide whether to continue betting that sanctions relief will persuade Mr. Maduro to allow a real vote, said Geoff Ramsey, a senior fellow for Venezuela at the Atlantic Council.“I think Maduro is really forcing Biden’s hand here,” he said. “It’s become clear that he can’t win a free and fair election, so he needs Washington to snap back the sanctions to justify a crackdown that allows the regime to revert to the status quo.”On Sunday, Venezuela held a referendum, backed by Mr. Maduro, on whether to annex the Essequibo region in Guyana. Mr. Maduro has cast the issue as a fight with ExxonMobil, the American oil company that has a deal with the Guyanese government. His critics say the vote was no more than a bid to divert attention from his political troubles by stoking nationalist fervor.Jorge Rodriguez, president of Venezuela’s National Assembly, with a map on Wednesday showing Essequibo as part of Venezuela.Pedro Rances Mattey/Agence France-Presse — Getty ImagesThe government reported a vote of more than 95 percent in favor. Though political analysts, social media users and New York Times journalists reported sparse turnout, the government claimed that it was heavy, with 10.5 million ballots cast.“With the inflated vote numbers, they’ve just become a mockery,” said Christopher Sabatini, a senior research fellow for Latin America at Chatham House, an international affairs research group in London. “Things really do seem to be falling apart.”The Essequibo region, with immense mineral and oil wealth but few people, is almost as large as Florida, taking up nearly three-quarters of the total area administered by Guyana. Venezuela and Britain both claimed it in the 19th century, and the dispute has continued since Guyana gained independence from Britain in 1966. The question is under consideration by the International Court of Justice in The Hague.At the same time that Mr. Saab was giving his news conference, Ms. Machado, a center-right former lawmaker, was holding one of her own at her party’s headquarters in Caracas, saying that the referendum had damaged the electoral authority’s credibility.As news of the charges and arrest orders spread on social media and through the room where Ms. Machado was speaking, her assistant pulled her campaign chief off the stage and whispered in her ear. Afterward, another party leader took the stage to say they were waiting for formal notice from the attorney general.The three party members who were charged left the headquarters without giving statements. They are the international relations coordinator, Pedro Urruchurtu; the political coordinator, Henry Alviarez, and the communications coordinator, Claudia Macero.The Americans accused by Mr. Saab are Damian Merlo, a consultant who has advised the authoritarian president of El Salvador, Nayib Bukele; and Savoi Jandon Wright. Mr. Saab gave no information about Mr. Wright, except that he was already imprisoned in Venezuela. More