Police said on Friday they had charged Slovakia's former president Andrej Kiska with tax fraud as the self-made millionaire is gearing up to hit the campaign trail ahead of February's general election.

Kiska is seen as a key opposition figure as surveys show his new "For the People" liberal party polling a close second behind the governing Smer Social Democratic party, led by his powerful arch-rival Robert Fico, himself indicted for hate speech.

The tax fraud charges stem from Kiska's successful 2014 presidential run, when he is alleged to have breached campaign spending rules by wrongfully using funds from one of his companies.

"The fraud is related to filing tax documents for the presidential election campaign on the company's accounts, while this activity was not the subject of its business," police said in a statement, without going into detail.

Kiska shot back, insisting in a Friday Facebook post that the charges were part of a "dirty political campaign and proof that there's a political mafia and blackmail ruling Slovakia".

Slovakian former president Andrej Kiska (pictured March 2019) has been charged with tax fraud stemming from his successful 2014 presidential run, when he allegedly misused funds from one of his companies
Slovakian former president Andrej Kiska (pictured March 2019) has been charged with tax fraud stemming from his successful 2014 presidential run, when he allegedly misused funds from one of his companies AFP / Michal Cizek

Although Kiska denied any wrongdoing in the case which first surfaced three years ago, he chose not to run for re-election as president in March, citing personal reasons.

Kiska handed Fico a stinging defeat in the 2014 presidential ballot.

Although Fico was then forced to step down as prime minister last year, after the murder of investigative journalist Jan Kuciak triggered mass anti-government protests, he is still widely seen as pulling the strings in Slovak politics.

Police charged Fico on Thursday with incitement to racial hatred for publicly approving hate speech by a far-right lawmaker against Slovakia's large Roma minority. Fico has denied any wrongdoing.

Support for his Smer-SD has dropped to the historic low of around 20 percent, but it is still poised to win the 2020 parliamentary election in the eurozone member of 5.4 million people.