President Hugo Chavez declared Thursday that it was "impossible" for him to lose a historic recall vote this weekend and charged that the United States was trying to undermine his embattled government.

In the hours after Chavez spoke, tens of thousands of people massed at a giant rally in the capital, Caracas, wrapping up a campaign to unseat the charismatic former army officer who launched an unsuccessful coup in 1992, then won the presidency in an election six years later.

"The advantage we have over the opposition is such that it's impossible there will be a surprise" in Sunday's vote, Chavez said at a news conference, noting that many opinion polls had put him in the lead. He added that if he did lose, he would immediately turn over the reins of government.

Both supporters and opponents of Chavez say the referendum could provide a peaceful, democratic solution to years of political turmoil in this South American country, the fourth-largest supplier of oil to the United States. But with many analysts predicting a tight race and both sides wary of voting fraud, a peaceful outcome is by no means certain.

The referendum on the president's rule, the first ever held in Latin America, was sought by a coalition of opposition forces that gathered millions of signatures. These groups accuse Chavez of weakening the country's democratic institutions and embracing communist Cuba.

For his part, Chavez says that the opposition has used undemocratic means to try to oust him -- first a short-lived coup in April 2002, then a general strike last year. Chavez declared Thursday that the Bush administration was behind the efforts to depose him and had continued to finance the opposition.

"Bush's government will be defeated on Sunday," the president told journalists. "The real confrontation in Venezuela isn't with the opposition. It has a master called Mr. George W. Bush."

The U.S. government has denied financing the opposition or otherwise trying to topple Chavez. But it clearly dislikes the Venezuelan leader. U.S. officials say Chavez has weakened his country's historically close relationship with Washington, providing little cooperation in areas such as terrorism or drug interdiction.

In addition, Chavez has embraced Cuban President Fidel Castro and expressed support for left-wing opposition movements in Latin America. U.S. officials have expressed concern that if Chavez wins Sunday's vote, he could become emboldened and turn into a destabilizing force in the hemisphere. Chavez has denied such a possibility.

While the two countries spar over foreign policy, they are hardly enemies. Chavez noted at the news conference that Venezuela continues to supply the United States with about 1.5 million barrels of oil a day. He also read from reports by several international brokerage firms praising his treatment of foreign investors. "We're not talking about Fidel Castro," the president said, brandishing a report from the noted Wall Street firm Lehman Brothers.

His opponents displayed equal optimism Thursday, the last day of official campaigning before the referendum. Tens of thousands of Chavez critics poured onto a Caracas highway, waving signs calling for a "Si" vote on ending the president's term immediately rather than in 2006, when it is due to end.

The mixture of people turning out illustrated how opposition to Chavez has grown beyond the country's wealthy and middle-class to encompass a wide range of Venezuelans, including poor people hit by the economy's drastic decline and leftists opposed to what they consider Chavez's authoritarian style.

Jean Yatim, 45, an employee of an oil multinational, said he disliked Chavez because his government had fomented class hatred and harassed members of the opposition, firing some from their jobs and denying others financial credits.

"We all agree there has to be more attention to social problems. But you can't make a more egalitarian society on the basis of violence and dividing the society," Yatim said.

Chavez has a sizable base of support among the country's poor, who have benefited from numerous new social programs, particularly as the price of oil has soared. Oil provides about half the government's revenues.

Venezuelan President Hugo Chavez faces a referendum Sunday on ending his term of office now rather than in 2006, as scheduled.At a mass rally in Caracas, tens of thousands of opposition supporters wrap up their campaign to unseat Chavez after six years as president.