Kazakhstan, the world's ninth-largest country by landmass, has attracted billions of dollars of Western, Russian and Chinese investment as production from its oilfields grows, but it has never held an election judged free and fair.
Zharmakhan Tuyakbai, an opposition challenger, told reporters after he voted in Almaty, the biggest city: "We already have the first alarming signals from the provinces where there have been a number of violations."
He said his campaign had evidence of duplicate voter lists that could allow multiple voting. These had been a feature of past elections where monitors reported voter list problems and pressure on state employees to vote for the government.
In the new capital, Astana, President Nazarbayev, 65, said he had wound up his own campaign early to give his opponents more of a chance. "This year's election is being held in unprecedented democratic conditions," he said.
The border with Kyrgyzstan
Onalsyn Zhumabekov, the head of the Central Election Commission, said the vote had conformed to Kazakh law.
Observers from the Organisation for Security and Cooperation in Europe were due to give their verdict on the conduct of the election at 4pm (1000 GMT) on Monday.
Early results from a Belarus-designed electronic voting system and traditional ballots were due to be announced after the count finished at 8am on Monday.
A Moscow thinktank that also provides public relations services, the Centre for Political Technologies, published an exit poll on its website (www.cpt.ru) giving Nazarbayev 87% of the vote. Two other exit polls by lesser known groups handed Nazarbayev 77% to 78%.
The opposition For a Just Kazakhstan alliance said it had no faith in exit polls "since most citizens are frightened of telling the truth and do not say who they vote for".
The alliance said it would not break the law by arranging demonstrations against alleged vote-rigging like those that swept through Ukraine, Georgia and Kyrgyzstan and ousted long-serving leaders.
But the authorities have taken no chances. They have closed the border with Kyrgyzstan and issued statements in recent weeks saying they would come down hard on any disorder.
Almaty, the most opposition-minded city, has been awash with rumours of impending unrest and Central Election Commission data at 6pm showed that it had a low turnout, 42% compared with the national average of 68%.
Bulat Abilov, an opposition campaign manager, said the low turnout could assist vote-rigging because it would leave unused ballot papers. "We have information that groups of people were bussed from one polling station to another," he said.