The Czech election, which began Friday afternoon and runs until early Saturday afternoon, is the country's first direct presidential election. Previous presidents were picked by parliament until public criticism over perceived insider deals prompted a switch in 2012.
Czech presidents exercise little day-to-day governance but do select prime ministers, judges and central bankers.
Some eight million Czechs facing recession are entitled to decide between the two advocates of European integration, who emerged from a first round of 16 candidates on January 11-12.
Surveys for the run-off indicate a narrow race. One late scan by the PPM Factum agency put Zeman ahead at 54 percent, with Schwarzenberg at 46 percent. The polling agency had erred during the first round by putting Schwarzenberg fourth.
Campaigning became inflamed over the past week.
Schwarzenberg, a 75-year-old prince, with a strong online campaign, slammed former Czechoslovakia's post-war expulsion of three million so-called Sudeten Germans, which he termed "collective punishment" by the-then Czech president Edvard Benes.
Zeman, 68, who was premier from 1992 to 2002 under a power-sharing deal with Klaus, blamed Schwarzenberg for painful austerity cuts made by the current government of Prime Minister Petr Necas.
The former communist described the Benes expulsion decrees during a television debate as an "indivisible part of Czech law."
Klaus, whose presidency expires on March 7, joined the fray, claiming Schwarzenberg was not Czech enough because he had lived in exile in Austria from 1948 until 1989.
Prominent Czech economist and author Tomas Sedlacek said the campaign had turned into mudslinging. "The atmosphere has flipped almost completely," he said.
Political analyst Josef Mlejnek of Prague's Charles University said the race would be "very tight."
"Milos Zeman is addressing voters from the lower-income groups, older and less educated, while Karel Schwarzenberg is attracting urban voters who are younger and better educated," said Mlejnek.
Cold War exile
Schwarzenberg, who was born into a family with large tracts of land, returned from exile during former Czechoslovakia's transition to democracy in 1989 and worked as chancellor under the playwright-turned anti-communist dissident Vaclav Havel, who died in 2011.
Czechoslovakia broke into Slovakia and the Czech Republic in 1993. In 2003, Havel was replaced by Klaus, who was re-elected Czech Republic president in 2008.
The nation of 10.5 million, which is heavily reliant on car exports, joined NATO in 1999 and the EU in 2004, but it is not a member of the eurozone. It has its own currency, the Czech koruna [crown].
Unemployment stands at 9.4 percent. Economic contraction last year was about one percent.
ipj/hc (AFP, dpa, Reuters)