Defense Minister Henry Rangel Silva said as he voted that all had been calm in the morning and he hoped that would continue. He said if any groups try to "disturb order, they should know there is an armed force prepared and equipped and trained... to put down any attempt at disturbances."
He didn't identify the groups to which he was referring.
Chavez held an impromptu news conference Saturday night, and when asked about the possibility of disputes over the vote, he said he expected both sides to accept the result. He says he has successfully emerged from about a year of cancer treatment.
"It's a mature, democratic country where the institutions work, where we have one of the best electoral systems in the world," Chavez told reporters at the presidential palace.
But he also said he hoped no one would try to use the vote to play a "destabilizing game." If they do, he said, "we'll be alert to neutralize them."
His opponents mounted a noisy protest in Caracas and other major cities on Saturday night, beating pots and pans from the windows of their homes to show displeasure with Chavez — and also their hopes for change. Drivers on downtown streets honked horns, joining the din.
The 40-year-old Capriles, a wiry former governor affectionately called "Skinny" by supporters, has infused the opposition with new optimism, and opinion polls pointed to him giving Chavez his closest election.
Some recent polls showed Chavez with a lead of about 10 percentage points, while others put the two candidates roughly even.
"Chavez is going to fight until his last breath. He doesn't know how to do anything else," said Antonio Padron, a bank employee backing the president.
Padron expressed optimism that the 58-year-old Chavez would win, noting the leader has survived a fight with cancer that has included surgery, chemotherapy and radiation treatment.
But Padron predicted a close finish: "It's a tough fight. The opposition has never been this strong."
Chavez won the last presidential vote in 2006 with 63 percent of the vote.
A former army paratroop commander first elected in 1999, Chavez has presided over an oil boom and has spent billions of dollars on government social programs ranging from cash benefits for single mothers to free education.
But he has suffered declining support due to one of the world's highest murder rates, 18 percent inflation, a deteriorating electrical grid and a bloated government accused of endemic corruption and mismanagement.
While his support has slipped at home, Chavez has also seen his international influence ebb since he emerged in the mid-2000s as leader of a like-minded club of newly elected Latin American leftist presidents.
"I want to tell President Chavez, I want to tell him his cycle is over," Capriles said at his final campaign rally Thursday.
Capriles says Chavez has stirred up hatred, hobbled the economy by expropriating private businesses and squandered oil wealth. He criticized Chavez's preferential deals supplying oil to allies, including one that lets Cuba pay with the services of Cuban doctors.
"We aren't going to finance the political model that exists in Cuba," Capriles said in a TV interview last week. "But we aren't going to break off relations with Cuba."
Chavez accumulated near-absolute power over the past decade thanks to his control of the National Assembly, pliant institutions such as the Central Bank and friendly judges.
Gino Caso, an auto mechanic, said he would vote for Capriles because he thought Chavez was power-hungry and out of touch with problems such as crime. He said his son had been robbed, as had neighboring shops.
"I don't know what planet he lives on," Caso said, gesturing with hands blackened with grease. "He wants to be like Fidel Castro — end up with everything, take control of the country."
Political analyst Ricardo Sucre said he expected the election to show "two halves, more or less even." Regardless of the result, he said, Venezuelans are likely to remain deeply divided by politics for years to come.
Associated Press writers Frank Bajak, Christopher Toothaker, Jorge Rueda and Vivian Sequera contributed to this report.
Ian James on Twitter: http://twitter.com/ianjamesap
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