Slovaks are voting in an election to decide if the controversial centre-left leader, Robert Fico, will remain in power for another four years.
Pre-vote opinion polls suggested Mr Fico's SMER party will win the vote but may not be able to form a majority coalition with its partners.
Growing public debt and a political fund-raising scandal may have dented his party's popularity.
Slovakia's worsening ties with Hungary also dominated the election campaign.
Earlier this month, a national newspaper published what it said was a recording of Mr Fico discussing improper party fund-raising.
Mr Fico said the audio was fake and is suing the SME daily's editor-in-chief.
In a television address shortly before polls opened, Mr Fico said that thanks to the underhand practices of his political opponents, Saturday's election would be undemocratic and manipulated.
Opposition parties have denied allegations of illegal smear tactics.
