Although voters appeared to spurn Smer a year after the murder of a journalist which sparked mass protests, the party's Maros Sefcovic, an EU Commissioner, could pick up support among those who prefer stability to the change offered by Caputova.

The killing of Jan Kuciak, who reported on fraud cases involving politically connected businessmen, triggered the biggest anti-government protests in Slovakia since communism ended three decades earlier. It also led to the resignation of then prime minister, Smer leader Robert Fico.

The government remains in power, but Smer's popularity has slumped. On the first anniversary of Kuciak's murder, thousands of Slovaks rallied to protest against what they see as a lack of government action on the corruption he uncovered.

With results from 99.4 per cent of polling stations counted on Saturday, Caputova was in pole position with 40.5 per cent of votes, far ahead of Sefcovic, who had 18.7 per cent.

As no candidate secured a majority of all votes cast in the European Union and NATO member country of 5.4 million, the pair will now contest a second round on March 30.

If elected, the 45-year-old Caputova, a pro-European liberal who belongs to the small, non-parliamentary Progressive Slovakia party, will stand out among the populist nationalist politicians on the rise across much of Europe.

Caputova has promised to end what she calls the capture of the state "by people pulling strings from behind," while maintaining the course of Slovakia's foreign policy.

"I see the message from voters as a strong call for change," Caputova told reporters early on Sunday after her victory in the first round was clear.

Australian Associated Press