A loss for Zeman to any of his main challengers could mean Czech presidency positions may shift closer to the European Union mainstream, in contrast to fellow ex-communist neighbours such as Poland and Hungary whose governments have clashed with Brussels.
Zeman was elected to the largely ceremonial post in 2013 during the country's first direct presidential vote, a victory that returned the former left-leaning prime minister to power. The man now in the job - Milos Zeman - known for his outspoken anti-immigration views, is leading opinion polls. After Zeman entered a polling station in Prague, he was approached by a female Femen activist stripped to the waist who shouted "Zeman, Putin's slut!"
Shouting the same slogan, the woman ran towards Zeman, who is running for a second term, in the polling station in the Czech capital, Prague, before being quickly wrestled to the ground by Zeman's bodyguards.
First-round voting ends at 2:00 pm (1300 GMT) on Saturday, and results are expected later in the afternoon.
Zeman, leading polls, is expected to fall short of winning over 50 percent of the vote and may face a strong challenger in a run-off on January 26-27. He regularly holds meetings with crowds in smaller towns.
Prague voter Martin Sauta said he voted for Zeman as he has "the most consistent views", while his rivals are "completely shapeless".
Zeman should do well in the first round due to fragmentation among opposition candidates sharing the same generally pro-EU platform.
Opinion surveys show the divisive 73-year-old ex-communist, who is also staunchly anti-Muslim and pro-Chinese, leading the pack of nine candidates ahead of Jiri Drahos, 68-year-old pro-European former head of the Czech Academy of Sciences.
Zeman has the backing of embattled billionaire Prime Minister Andrej Babis, who said the incumbent "fights for our national interests" but called on him to "unite, not divide" Czechs.
Zeman once called the 2015 migrant crisis "an organized invasion" of Europe, and has said that Muslims were "impossible to integrate".
The CTK news agency pegged turnout at 40 percent after day one of voting.
The two are among the most popular politicians in the country, whose 10.6 million people are widely eurosceptic and reject accepting migrants from the Middle East and Africa.
"It is a clash between... the post-communist part of society represented by Zeman and the other part, say, modern, pro-Western, which simply doesn't want this president any more", he told AFP. "Data also show a deepening rift between cities and the countryside".