By JUAN FORERO
 
CARACAS, Venezuela, Aug.16 — Venezuelans have voted to keep Hugo Chávez as their president, electoral authorities and international observers said today. But a vocal opposition movement refused to accept that a vast majority of the electorate sided with the firebrand leader in a recall election the day before.

The National Electoral Council president, Francisco Carrasquero, announced at 4 a.m. that Mr. Chávez had won in a landslide after 18 hours of voting on Sunday, securing the backing of 58 percent of voters compared to 42 percent against him.

"There is a clear difference in favor of the government of President Chávez," former President Jimmy Carter said. Mr. Carter, who heads the Atlanta-based Carter Center, which monitored the election with the Organization of American States, held a joint press conference with the OAS's secretary general, César Gaviria.

In the packed conference, shown on national television, the two men explained that the highly accurate "quick counts" their organizations conducted at various polling stations coincided with the outcome released by the Electoral Council.

The quick counts, used in elections around the world, tally up totals from various polling sites and have a margin of error of 1 percent.

"We have found the information from the quick count was almost exactly the same as that presented" by the electoral authorities, Mr. Carter said. The Nobel Peace Prize winner has monitored elections in 50 counties. He added that "all Venezuelans should accept the results of the CNE," the electoral body, "unless there is tangible proof that the reports are incorrect."

As of 3 p.m., the opposition movement, whose leaders had said last week that they would accept results as long as they were confirmed by the monitors, is clinging to the charge that a "gigantic fraud" has been committed.

Shortly after the polls closed at midnight, opposition leaders had giddily predicted victory. When informed in a two-hour meeting with Mr. Carter and Mr. Gaviria before dawn that they had lost, the leaders were stunned, according to people who were present.

"We categorically reject the results," said Henry Ramos, spokesman for the Democratic Coordinator, the umbrella of 27 political parties that opposes the government. In a televised announcement soon after the meeting, he said: "They have perpetrated a gigantic fraud against the will of the people."

Mr. Carrasquero said the government mustered 4,991,483 votes, while the opposition collected only 3.6 million votes, which was 200,000 short of what they would have needed to actually recall the president.

The latest polls, completed in July, had shown the president might squeak to victory, but political analysts had declined to predict an outcome, saying it was too close to call. As it turned out, Mr. Chávez's fervent supporters swamped the polls and voted overwhelmingly for him.

Mr. Carter and Mr. Gaviria said that there could be some discrepancies but said fraud was all but impossible. They said that they would look into fraud claims, if tangible proof were presented.

But the two former presidents — Mr. Gaviria from Colombia — cast doubt on fraud claims, saying that in addition to their quick counts, the opposition group, Sumate, had come up with similar results in their own samplings.

"We're willing to work with the opposition, if there are precise complaints," said Mr. Gaviria. "We haven't yet found any elements of fraud."

Opposition leaders, reached this morning by phone, insisted that the new computerized voting system had been tampered with, permitting the government to lose. But the O.A.S. and the Carter Center said that the results could not have been manipulated.

Antonio Mugica, the owner of the Smartmatic voting machines used in the election, said: "We know that it's impossible. All the machines were O.K. The votes were recorded in all the machines" and were then transmitted to the council.

He added: "The servers are in a highly secure area. The system is impenetrable."

Mr. Chávez, holding a microphone and standing in a balcony of the Miraflores presidential palace, spoke to a throng of supporters in a predawn address, telling them that the "the Venezuelan people have spoken."

"The Venezuelan people have spoken and the people's voice is the voice of God!" Mr. Chávez said. He was also conciliatory toward the opposition, which he has called "squalid ones" and a "rancid oligarchy" in the past.

"This is a victory for the opposition," the president said. "They defeated violence, coup-mongering and fascism. I hope they accept this as a victory and not as a defeat."

Bloomberg News reported that crude oil futures fell from record highs after the vote was announced. There had been concerns in the oil markets that a defeat would have disrupted supplies from this country, the world's fifth-largest exporter of oil and a key supplier to the United States.

Brent crude oil for September delivery, which expires today, fell as much as 58 cents, or 1.3 percent, on London's International Petroleum Exchange and closed down 21 cents at $43.67. On the New York Mercantile Exchange, crude oil for September delivery closed at $46.05, down 53 cents a barrel.

With about 60 percent of Venezuelans voting, the results appear to permit Mr. Chavez to finish out the two years left on his tumultuous term, which began after he won re-election in 2000.