Venezuelans are voting in the country's presidential elections, with Hugo Chavez, the current president, seeking re-election.
Many Venezuelans support Chavez for spending oil income on social programmes, while critics accuse him of seeking to copy the communism of his ally, Fidel Castro, the Cuban president.
Chavez opponent
Rosales, governor of the western oil state of Zulia, has attacked Chavez's record on tackling high crime and unemployment. He has promised to stem policy which he says is edging Venezuela towards authoritarian rule.
"The future of Venezuela is at stake," Rosales told supporters shouting "President, President" as he cast his ballot in his home state.
"This is one of the most important elections in the history of our country," he said.
He appealed to the electoral authorities to mend some voting machines printing out blank ballots when voters selected "Rosales". Chavez has said that Rosales will look for an excuse to cry fraud.
"There is no possibility of any fraud in the election," electoral council official Vicente Diaz told the El Universal newspaper in an interview published on Saturday.
Divided country
Venezuela is sharply polarised. Many poor people applaud Chavez's spending of oil income on health and education while some upper- and middle-class voters say he is a fledgling dictator determined to follow Castro.
"There is no possibility of any fraud in the election" Vicente Diaz, electoral council official Send us your viewsThe most recent opinion survey, commissioned by the state oil company with a US pollster, showed Chavez with a 19-point lead over Rosales.
"There is no possibility of any fraud in the election"
Vicente Diaz, electoral council official
Send us your views
Rosales has mustered the opposition's most serious challenge to Chavez in years with populist promises to redistribute Venezuela's oil wealth and roll back policies he says are edging the country towards Cuban-style communism.
A Chavez victory would shore up his campaign to forge a regional alliance of leftist leaders to counter Washington's influence in Latin America. He applauded this week's victory by leftist Rafael Correa in Ecuador's presidential run-off.
A former paratrooper who led a botched coup six years before his 1998 election, Chavez says he is inspired by South American liberation hero Simon Bolivar to free Latin America from US-backed "imperialist" policies.