This bitterly divided nation votes Sunday on whether to recall President Hugo Chavez, with both sides warning of possible fraud and the government saying that Chavez's defeat could produce turmoil in the oil industry, a key supplier of the U.S. market.

International observers said they expect the referendum to be generally clean and the result to be respected. But the volley of accusations and threats of unrest reflect the country's deep polarization over Chavez, a former paratrooper who has upset the middle class and U.S. government with his leftist, populist rule.

The referendum will decide whether the president leaves office two years before the end of his six-year term. Pollsters have said the vote could be close.

Chavez, 50, has pledged to step down if he loses and compete in a new presidential election that would follow in a month. But he also has declared it "impossible" for him to be defeated in the referendum. On Friday, one of his top aides suggested that Chavez's removal could plunge the country's oil industry into turmoil, at a moment when world petroleum prices are surging to record levels.

"There is no way our people, and this includes the oil workers, would accept a defeat" of Chavez, said Rafael Ramirez, minister of energy and mines. The referendum is intended to resolve years of clashes between Chavez and a broad range of opponents who accuse him of authoritarian rule. The conflict has at times exploded into violence, with the opposition staging a coup attempt in 2002 and later a three-month general strike. But Chavez has remained popular with many of this South American country's poor, whom he has rewarded with social programs and a sense of greater political power.

The vote is of keen interest to the United States, which counts Venezuela as its fourth largest source of petroleum, after Canada, Saudi Arabia and Mexico. The Bush administration has been irritated by Chavez's fervent verbal attacks and his embrace of Cuba's Fidel Castro and leftist movements.

Venezuela is one of the oldest democracies in Latin America; its last dictatorship ended in 1958. But the country is now so bitterly divided that many people here fear Sunday's referendum could be followed by a violent outburst, particularly if the losers cry foul.

Sunday, voting day, "is very important. But the 16th is even more important," said Elias Santana, a grass-roots activist working with the opposition.

Chavez's opponents, who include politicians, business executives and most of the country's news organizations, have accused the government of numerous irregularities leading up to the vote. They allege, for example, that authorities have switched thousands of registered voters to polling places far from their homes. And they are wary of new election technology making its debut, including touch-screen voting machines and equipment to capture fingerprints.

The national election commission, which oversees the vote, "can't be trusted," charged Enrique Mendoza, the leading figure in the Democratic Coordinator, the opposition coalition. He noted that Chavez supporters hold a majority on the five-member panel.

"If we lose in transparent elections, we'll recognize that. But we won't accept tricks," Mendoza said at a news conference Friday.

Chavez, who led an unsuccessful coup in 1992 before being elected president in 1998, has promised a fair vote. He has criticized his opponents for failing to respect his numerous electoral victories, including the landslide win that brought him to office, the approval of a new constitution in 1999, and his reelection to the presidency in 2000 under the new constitution.

International observers said they have detected no systematic fraud leading up to the vote and have confidence in the new voting machines. While Chavez's government was initially reluctant to allow international observers to freely monitor the balloting, it has backed down and quietly agreed to many of their requests, diplomats said. A team from the Organization of American States (OAS) and another led by former president Jimmy Carter will be present.

"The agreements between the government and the opposition have brought a more credible electoral system," Cesar Gaviria, secretary general of the OAS, told journalists on Friday night. Because of such credibility, he said, widespread violence was unlikely.

The opposition is suspicious of Sunday's vote in part because of the government's repeated challenges of the referendum process. The opposition gathered millions of signatures to hold the recall, but the Chavez administration alleged fraud and harassed some of the people who signed the petitions, according to diplomats and government critics.

But faced with international pressure and the specter of street violence, Chavez ultimately agreed to the referendum. As the vote approached, his popularity rose as he used soaring oil income to launch education, health and food programs in this nation of 25 million.

Chavez has also played on fears of instability if he loses the recall. Millions of people in this poor country are passionately dedicated to the charismatic president. He is believed to have strong support in the military and in the state-run oil company, which he purged of opponents after they spearheaded a general strike aimed at forcing his resignation.

Still, for all the doomsday scenarios, diplomats and Chavez aides said his supporters would likely fall in line behind him if the president loses and accepts the results.

"We have a leader. His name is Hugo Chavez," said one of his top campaign aides, Samuel Moscada. If Chavez is recalled, he said, "it's not a catastrophe. In 30 days there will be another election."

If Chavez wins on Sunday by a slim margin, however, "it would be very tense," Moscada said. "This would be almost as bad as if he lost."

Many polls gave Chavez a small lead going into the referendum. However, with about 10 percent of likely voters saying they were undecided, the result was difficult to predict, pollsters said.

The opposition coalition has mobilized hundreds of thousands of people, ranging from right-wing businessmen to homemakers to former leftist guerrillas. Analysts said the opposition has suffered from a lack of leadership and a clear program but has considerable experience in mobilizing voters.

Chavez has also rallied hundreds of thousands of followers to promote his cause. He has campaigned with the slogan "No to the Past," referring to former politicians associated with the upper class who were reviled for their poor economic results and alleged corruption.

"A year ago, we thought the only way Chavez could win was through cheating," said Luis Vicente Leon, director of the Datanalisis polling firm, noting that the president's popularity had dipped at that time because of a prolonged recession. But now the economy has started to grow, and Chavez has run an effective campaign, putting him on an equal footing with his opponents, analysts said.

His message has convinced Marta Vielma, 41, who sells beauty creams from her home in a working-class area of Caracas.

"The only president who has done things for the people is Hugo Chavez," she said. "All those people who ruled before just destroyed the country."

Venezuelans in Caracas look at voting lists to learn where to cast ballots Sunday in a recall referendum on President Hugo Chavez.Venezuelan President Hugo Chavez talks to the media at the Miraflores Palace in Caracas. He said it is "impossible" for him to be defeated in the referendum.