CARACAS, Venezuela — President Hugo Chavez would take on expanded powers and no longer face term limits under sweeping constitutional changes being considered by Venezuelans Sunday in a contentious vote that has raised tensions in South America's top oil exporter.

An emboldened opposition and recent violent clashes involving protesters point to a potentially volatile dispute if the vote is close, as some pollsters predict.

Lines snaked outside polling stations as voting began. Voters were awakened in Caracas by fireworks exploding in the pre-dawn sky and reveille blaring from speakers mounted on cruising trucks.

"This is transcendental day for Venezuela," voter Raul Perez said, without revealing whether he was voting "yes" or "no."

Chavez has warned opponents he will not tolerate attempts to stir up violence, and threatened to cut off oil exports to the U.S. if Washington interferes. His country is a major supplier to the United States, which in turn is the No. 1 buyer of Venezuelan oil.

"In the case of an aggression by the United States government, we wouldn't send any more oil to that country," Chavez told reporters Saturday. "Forget about our oil."

Chavez, who has become Latin America's most outspoken antagonist of Washington since he was first elected in 1998, calls the constitutional overhaul vital to making Venezuela a socialist state. He labels those who resist it pawns of U.S. President George W. Bush.

While the Venezuelan government touts polls showing Chavez ahead, other surveys cited by the opposition indicate strong resistance — which would be a change for a leader who easily won re-election last year with 63 percent of the vote.

Pollster Luis Vicente Leon said tracking polls by his firm Datanalisis in the past week show the vote is too close to predict. Which side wins will depend largely on turnout among Chavez's supporters and opponents, he said.

"If he wins by a very small margin, that's a scenario filled with conflict," Leon said. "In a country where there are high levels of mistrust between the camps, it's obvious the opposition ... would think it was fraud."

In Washington, State Department spokesman Sean McCormack said the United States hopes the referendum will be "a free and fair contest."

Speaking to reporters Saturday, Chavez accused the U.S. government of plotting to discredit what he says will be a legitimate victory for him at the polls.

"They are preparing to disavow the results, so we hope the popular will is respected, whatever it is," Chavez said. "The government of the United States is a threat."

Chavez thanked his ally, Nicaraguan President Daniel Ortega, for recently "alerting the world to the plan Bush's government has to kill the president of Venezuela." He didn't offer specifics but warned that any assassination attempt would lead to "events that aren't very good for the United States or for the world."

Chavez often makes such accusations, which U.S. officials deny.

The socialist leader has sought to capitalize on his personal popularity ahead of the vote. He is seen by many supporters as a champion of the poor who has redistributed more oil wealth than any other leader in memory.

Opponents — including Roman Catholic leaders, press freedom groups, human rights groups and prominent business leaders — fear the reforms would grant Chavez unchecked power and threaten basic rights.

The changes would create new forms of communal property, extend presidential terms from six to seven years and let Chavez seek re-election in 2012 and beyond.

They would also grant Chavez control over the Central Bank, allow his government to detain citizens without charge during a state of emergency, and empower him to redraw the country's political map and handpick provincial and municipal leaders.

Many Chavez supporters say he needs more time in office to consolidate his unique brand of "21st century socialism," and they praise other proposed changes such as shortening the workday from eight hours to six, creating a social security fund for millions of informal laborers and promoting communal councils where residents decide how to spend government funds.

Tensions have surged in recent weeks as university students led protests and occasionally clashed with police and Chavista groups. One man was shot dead Monday while trying to get through a road blocked by protesters.

The opposition has called for close monitoring of an outcome they predict will be close.

Some 140,000 soldiers and reservists were posted for the vote, the Defense Ministry said.

About 100 electoral observers from 39 countries in Latin America, Europe and the United States will be on hand, plus hundreds of Venezuelan observers, according to the National Electoral Council.

Absent this time are the Organization of American States and the European Union, which have monitored past votes.

Chavez, 53, says he will stay in power only as long as Venezuelans keep re-electing him — and adds that might be for life.

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

Associated Press writers Christopher Toothaker, Sandra Sierra and Edison Lopez contributed to this report.