Fico, who took power for the fourth time last October, has turned the country's foreign policy to more pro-Russian views and initiated reforms of criminal law and the media, which have raised concerns over weakening the rule of law.

Pellegrini had 53.26% of the vote, versus 46.73% for pro-Western opposition candidate Ivan Korcok, results from 99.66% of voting districts showed.

Slovak presidents do not have many executive powers, but can veto laws or challenge them in the constitutional court. They nominate constitutional court judges, who may become important in political strife over the fate of Fico's reforms, which would dramatically ease punishments for corruption.

Fico's coalition that includes a party headed by Pellegrini halted Slovak official shipments of weapons for Ukraine and Fico has spoken about what he called western influence in the war which only led to Slavic nations killing each other.

Pellegrini, 48, said his victory meant the government would have support in its aims, and not face an "opposition, opportunistic power centre" in reference to outgoing liberal president Zuzana Caputova.

"I will be a president who will support the government in its efforts for improving people's lives," Pellegrini said at his campaign headquarters.

"I will do everything for Slovakia to forever remain on the side of peace and not the side of war."

Pellegrini has portrayed Korcok as a warmonger for his support for arming Ukraine and suggested he could take Slovak troops into the war in the neighbouring country, which Korcok denied.

Pellegrini, seen as more moderate than Fico, said earlier on Saturday his election would not mean a rush to any fundamental change in foreign policy.

"This is not about the future direction of foreign policy. I also guarantee, like the other candidate, that we will continue to be a strong member of the EU and NATO," he said after voting in Rovinka on the outskirts of the capital.

Korcok conceded defeat and congratulated Pellegrini, but lashed out at him for winning by spreading fear.

"A campaign can be won by making the other a candidate of war. I will not forget this," Korcok said at his campaign headquarters. "The decisive factor was high turnout, I respect that, but it was fear that decided...spreading fear and hatred."

Korcok, like Kyiv's western allies, had argued that a halt in supplying Ukraine would not lead to peace but to Russia's victory.

The independent Korcok, 60, was Slovakia's envoy to the EU and later ambassador to the U.S., before taking the foreign affairs portfolio in centre-right governments in 2021-2022.

At the time, Slovakia was a staunch ally of Ukraine, providing it with air defence and fighter jets.

Pellegrini, now speaker of parliament, was a long-time ally of Fico, who picked him to be prime minister after Fico was forced to resign amid public protests against corruption following the murder of an investigative journalist in 2018.

He later split from Fico to set up his own party, Hlas (Voice), more centrist and liberal than Fico's populist-leftist SMER-SSD, but formed a government with Fico and the nationalist SNS last October.