Hugo Chavez, Venezuela's president, is facing his toughest political challenge in years as voters decide whether to approve constitutional changes that would greatly expand his powers.
Projections suggested that the result was too close to call as Venezuelans went to the polls on Sunday.
An emboldened opposition and recent violent clashes during street protests have created fears of a potentially volatile dispute.
Chavez has warned opponents he will not tolerate attempts to stir up violence, and threatened to cut off all oil exports if the US interferes in the referendum.
He said on Sunday: "There will be no oil for anyone, and the price per barrel will go up to 200 dollars."
The US remains the biggest purchaser of Venezuelan oil.
Lucia Newman, Al Jazeera's correspondent in Venezuela, said: "If approved, these constitutional reforms will not only eliminate presidential term limits, but they would also give president Chavez enough power to transform the political, enconomic and social structure of Venezuela into what he calls a truly socialst, revolutionary state.
"There are a lot of people, even those who have traditionally supported president Chavez, who fear he may be going too fast too far."
Chavez has said the changes are vital for his plans for "21st century socialism", and labelled those who resist it pawns of George Bush, the US president.
Opponents, including Roman Catholic leaders, press freedom groups, human-rights groups and prominent business leaders, fear the changes would grant Chavez unchecked power and threaten basic rights.
At the very least, they would grant Chavez direct control over the central bank, allow his government to detain citizens without charge during a state of emergency, and empower the president to redraw the country's political map and handpick provincial and municipal leaders.
Other proposed changes - such as shortening the workday from eight hours to six, creating a social security fund for millions of informal labourers and promoting communal councils where residents decide how to spend government funds - have been welcomed by supporters.
While the government has touted polls showing Chavez ahead, other surveys cited by the opposition have indicated strong resistance, which would present a challenge for a leader who won re-election last year with 63 per cent of the vote.
Luis Vicente Leon, a pollster, 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."
The opposition has called for close monitoring of the results, raising tensions before a vote on changes that would extend presidential terms from six to seven years, create new forms of communal property, and let Chavez seek re-election in 2012 and beyond.
About 100 electoral observers from 39 countries in Latin America, Europe and the US are on hand, plus hundreds of Venezuelan observers, the National Electoral Council said.
Chavez has accused his opponents of plotting to discredit the legitimate results of what he says will be a "knockout" at the polls, saying his enemies enjoy support from Washington.
Manuel Rosales, a Venezuelan opposition leader and the governor of Zulia state, recalled before a crowd of supporters on Friday night that he conceded defeat in the 2006 presidential race and urged Chavez to do the same and "recognise the will of the people".
Güncelleme Tarihi: 03 Aralık 2007, 12:22
Al Jazeera, Agencies