President Hugo Chavez would take on expanded powers and have a shot at being president for life under constitutional changes being decided by Venezuelans Sunday in a contentious vote that raised tensions in South America's top oil exporter. Lines snaked outside many polling stations, and Information Minister Willian Lara said across the country there was "a massive turnout." An emboldened opposition and recent violent clashes involving protesters point to a potentially volatile dispute if the vote is close, as some pollsters predict. Voters were awakened in Caracas by fireworks exploding in the pre-dawn sky and reveille blaring from speakers mounted on cruising trucks. "This is a 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 US 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. 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 US President George W. Bush. Venezuelans across the political spectrum saw the referendum as a turning point. Some Chavez opponents described it as a protest vote - and a point of no return. "This is our last chance to change things," said Judith Padova, a 57-year-old housewife who lined up among about 300 voters in the Caracas neighborhood of Los Ruices. 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. Opposition leader Manuel Rosales, who lost to Chavez in the 2006 presidential race, urged voters to turn out in large numbers. "Venezuela is in the middle of a great crossroads," Rosales said before casting his ballot in western Zulia state, where he is governor. His supporters chanted, "freedom, freedom!" Before the vote, US State Department spokesman Sean McCormack said Friday that the United States hopes the referendum will be "a free and fair contest." Speaking to reporters Saturday, Chavez accused the US 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," Chavez said. The socialist leader sought to capitalize on his personal popularity ahead of the vote. He is seen by many 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. If Chavez were to lose, he would be barred from running for re-election in 2012. The reforms 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 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. Meetings and demonstrations were prohibited on election day as usual, said Tibisay Lucena, chief of the National Electoral Council. She called the vote "the calmest we've had in the last 10 years." About 100 electoral observers from 39 countries in Latin America, Europe and the United States were on hand, plus hundreds of Venezuelan observers, the electoral council said. Absent were 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 - but has added that might be until 2050, when he would be 95 years old.