Government critic Zuzana Caputova secured a huge first-round win over the ruling party candidate in Slovakia's weekend election, putting her one step closer to becoming the country's first female president.

Provisional results showed Ms Caputova securing more than twice the votes of her rival Maros Sefcovic, suggesting the electorate had shunned the political establishment following last year's shock murder of an investigative journalist probing corruption.

The liberal environmental lawyer, who secured 40 percent of the vote, is almost certain to win the March 30 run-off against European Commission vice-president Mr Sefcovic, who took just 19 percent.

The outcome is likely to spell trouble for the governing Smer-SD party ahead of next year's general election.

"Caputova has such a large lead that Sefcovic would have to attract nearly all the unsuccessful candidates' voters and that is improbable," Bratislava-based analyst Grigorij Meseznikov said.