Venezuela’s authoritarian leader, Nicolás Maduro, was declared the winner of the country’s tumultuous presidential election early Monday, despite enormous momentum from an opposition movement that had been convinced this was the year it would oust Mr. Maduro’s socialist-inspired party.

The vote was riddled with irregularities, and citizens were angrily protesting the government’s actions at voting centers even as the results were announced.

With 80 percent of voting stations counted, the country’s election authority claimed that Mr. Maduro had received 51.2 percent of the vote, while the main opposition candidate, Edmundo González, had received 44.2 percent.

Mr. Maduro’s government has invented election results before, and this tally was immediately called into question by the opposition and by several officials in the region.

“We won and the whole world knows it,” the country’s most popular opposition leader, María Corina Machado, told reporters in Caracas, the capital, early Monday. She called the declared result “impossible,” given information her team had collected about turnout.

Secretary of State Antony J. Blinken, speaking to reporters in Tokyo, said the U.S. government had “serious concerns that the result announced does not reflect the will or the votes of the Venezuelan people.”