Lukashenka, who has ruled Belarus for 12 years and is seeking a third term, says anyone who tries to seize power through protests will have his neck broken like a "duckling."
However, observers say the capital, Minsk, is calm today.
The candidates include Lukashenka, main opposition leader Alyaksandr Milinkevich, Liberal Democratic leader Syarhey Haydukevich, and Social Democratic leader Alyaksandr Kazulin.
Milinkevich on March 18 called for a street demonstration after the polls close, but the authorities have warned they will crush any attempt to stage a popular revolt.
A Soviet-Style Vote
Valery Karbalevich is an analyst with the Minsk-based Strategy Center for Political Analysis. He says the election is well organized and resembles a Soviet-era vote.
"Technically, the election is well organized," Karbalevich said. "People come, fill in the ballots, and vote. It surprises foreign observers. They say that they visited several polling stations and everything looks just fine; nobody is forced [to vote.] But in the Soviet Union everything also looked just fine [during elections.]"
High Turnout Expected
Turnout is brisk, according to the Central Election Commission, at just under 40 percent of registered voters after three hours. The results are valid once turnout reaches 50 percent.
Karbalevich says he believes it is not the voting process that matters, but how the votes are counted. He says the main events are likely to happen later this evening, when the opposition is expected to rally on the streets
Syarhey Vozniak, a spokesman for Milinkevich, says the authorities are already making preparations for this evening.
Authorities, Opposition Preparing For Demonstrations
"According to the information we have, they -- power structures -- will start blocking the center of the city in the afternoon," Vozniak said.
On March 18, Milinkevich urged supporters to gather in the streets after the polls close.
"Presidents are not elected on the streets, but we come out onto the streets to say 'no' to falsification, 'no' to lies," Milinkevich said. "We want to live in dignity. We are human beings."
Vozniak says tonight's rally is not planned as a protest demonstration. He says the people are being called to gather to hear the results.
"There, we will listen to the official results of the vote," Vozniak said. "According to our point of view, these results will be falsified."
He says the opposition wants a peaceful rally, but if violence erupts, he says it will be the authorities who are to blame.
The KGB security service says protesters could be charged with "terrorism."
On March 18, EU foreign policy chief Javier Solana said any violence against peaceful protests "would meet with a strong international reaction."
RFE/RL In Minsk
RFE/RL Belarus Service Director Alexander Lukashuk (RFE/RL)
VOTING UNDER WAY: RFE/RL's Belarus Service Director ALEXANDER LUKASHUK is in Minsk today (March 19) covering the vote. He spoke by phone with RFE/RL correspondent Valentinas Mite.
RFE/RL: How is the vote is going?
Lukashuk: The first thing which I learned today is that one-third of voters had already voted [before March 19] during the five days from Monday to Saturday. In the presidential elections in 2001, the number was only 14 percent of voters.
RE/RL: What is the significance of the early voting?
Lukashuk: [The significance is] that people are coming to the polling stations and just casting ballots. Of course, during this [stage of the] vote there are no observers and nobody is checking the polling boxes. And one-third of the population has voted this way.
RFE/RL: What do the exit polls say and which groups are conducting them?
Lukashuk: At 10 a.m., the first results of exit polls were announced and the results showed that 82 percent of voters had chosen Lukashenka. [Two organizations are allowed] to conduct exit polls in the country: the Union of Belarusian Republican Youth and an organization called EKOOM. These organizations are quasi-state-owned. These exit polls mean nothing. The figures are only for propaganda purposes."
RFE/RL: Have you heard reports that further arrests of opposition figures are under way, following the arrests in the run-up to the polls of many others?
Lukashuk: Yes, the arrests of activists supporting [opposition] candidates are continuing. [More arrests happened today] in Brest, Mohilev, and Minsk.
MEET THE CANDIDATES: Read brief biographies of the four candidates in the March 19 election.