Nazarbayev, a 70-year-old former Communist party boss, has ruled the oil-rich Central Asian nation virtually unchallenged since the 1980s, when it was still part of the Soviet Union, and the opposition is refusing to take part in this vote, which they call a sham.

In the freezing wind-swept capital, Astana, which was blanketed in a layer of snow overnight, voters backing Nazarbayev echoed familiar mantras of stability.

“I have performed my civic duty, I have voted for Kazakhstan,” said 30-year-old security guard Bolat Salykov, quoting from the slogan of a concerted get-out-the-vote drive that preceded the election.

Although Nazarbayev’s victory is all but a foregone conclusion — by some estimates, he may garner some 90 percent of the vote — authorities have demonstrated an evident nervousness that the final turnout could be lower than hoped. Leading opposition politicians have refused to take part and have urged their supporters to boycott the election.

But with more than 40 percent of the 9 million eligible voters casting their ballot by midday, the turnout looked certain to pass the 50 percent minimum required threshold.

Preliminary results are due to be announced Monday morning.

Anti-election videos have proliferated online and been passed around by e-mail. That grassroots approach has proved little match, however, for the slick pro-vote “For Kazakhstan” campaign fronted by television personalities and pop singers.

Also, several reports have surfaced of state university students being pressured into voting or risk being expelled.

A scanned letter distributed by e-mail this week apparently signed by the rector of Kainar University in the financial capital, Almaty, warns that college deans would incur “severe punishments for the failure of presidential elections.” The Organization for Security and Cooperation in Europe’s election monitoring arm registered its concerns in pre-vote reports over a lack of transparency and the feeble spirit of competition demonstrated by the candidates.

Most campaign posters visible in the main cities belong to the Nazarbayev campaign, and the president has as usual dominated the daily television news headlines. The three nominal opposition candidates have made little impression, and even those that have heard of them appear not to have been tempted to give them their preference.

“I have voted for the leader of our nation. ... I know the other candidates all too well to want to vote for them,” said hospital worker Aigerim Tulegenova, 27.

In a small protest outside the election commission building in Almaty, a group of demonstrators set up their own ballot box and cast their vote “against all.” That option, which was last legally available in the 1999 presidential election, was not included on ballot papers this time.

Nazarbayev arrived at a polling station near the presidential palace together with his wife and several members of his extended family, including billionaire son-in-law Timur Kulibayev.

He thanked Kazakhs for participating in the election, saying it marked an achievement in the nation’s history as an independent state.

“The task of modernizing the state and society are still huge, so today’s vote will determine our unity and our desire to fulfill our plans,” he said.

Nazarbayev’s term was to have ended in 2012, but he called the early election in January after a proposed referendum on canceling the next two elections was ruled unconstitutional.

He has said he made the call to ensure that the people retained their trust in democracy, but critics speculated he was trying to head off any popular uprising like those that were beginning to sweep the Middle East and North Africa.