Speaking to more than 200,000 supporters, Chavez warned that his opponents at home could try to sabotage the vote with backing from Washington through violent protests on the night of the vote.

"If 'Yes' wins on Sunday and the Venezuelan oligarchy, the violent Venezuelans - the ones who play the (U.S.) empire's game - unleash violence with the tale that there was fraud ... minister, that very Monday you order a halt to the shipments of oil to the United States," Chavez said, addressing his oil minister, Rafael Ramirez.

Venezuela was the fourth largest oil exporter to the United States in 2006.

Halting exports to the U.S., the No. 1 buyer of Venezuelan oil, would cut off a major source of income for the Venezuelan government.

Chavez dismissed Venezuelans who oppose the constitutional changes as beholden to U.S. interests.

"Anyone who votes 'No' is voting for George W. Bush," he said. "Our real enemy is called the U.S. empire, and on Sunday, Dec. 2, we're going to give another knockout to Bush, so no one forgets that is the battlefield."

Chavez's opponents have called for close monitoring of results in what they expect to be a tight contest, raising tensions ahead of a vote on sweeping changes that would left Chavez seek re-election in 2012 and indefinitely.

"If God gives me life and help," Chavez said, "I will be at the head of the government until 2050!" - when he would be 95 years old.

"To the Venezuelan oligarchy and the U.S. empire, from here I'm warning them that they won't be able to stop the car of the Bolivarian Revolution, because on Sunday we will approve the constitutional reform," Chavez said.

There were no independent crowd estimates, but reporters estimated the crowd at more than 200,000.

The government cites polls showing Chavez leading ahead of the referendum, while other polls have predicted a close race.

Pollster Luis Vicente Leon, whose firm Datanalisis found the "no" option leading in a poll earlier this month, said Friday that two other later tracking polls by his firm found Chavez had closed the gap and the two sides were statistically about even.

"We don't know who's going to win," Leon said. "The result will depend on the level of abstention that ends up happening. Whoever has the greatest weight to achieve turnout among their voters at the polls is going to win."

In Washington, State Department spokesman Sean McCormack said the United States hopes the referendum will be "a free and fair contest in which the will and desire of the Venezuelan people is reflected."

The Venezuelan Embassy in Washington responded with its own release, saying McCormack's statements "are aimed at generating doubts regarding our electoral branch and asserting that there are no guarantees that the result of the elections expresses the will of the majority of the people."

Chavez also threatened to expel journalists for the CNN international news network if they assisted in any plot to overthrow his government.

If CNN "came here to lend its correspondents to an imperialist operation, they will be thrown from the country," Chavez said.