Peter Marki-Zay, a conservative provincial mayor, looks set to challenge Hungarian Prime Minister Viktor Orban at an election next year after taking a lead in an opposition primary vote Sunday, according to partial results.

Marki-Zay, a practising Catholic and father-of-seven, led Klara Dobrev, an MEP with the leftist Democratic Coalition party (DK) by a margin of 60-40 percent in a second-round run-off, with around half the results declared.

The primary was designed by a six-party opposition alliance formed last year in an effort to combat a mainly first-past-the-post election system that favours Orban and his ruling right-wing Fidesz party.

The vote, Hungary's first ever primary contest, aimed at selecting just one contender to oppose Orban -- as well as single candidates in each constituency to go up against Fidesz -- in the election due next April.

Opinion polls have put the opposition alliance neck-and-neck with Fidesz, and Marki-Zay best-placed to defeat the nationalist premier.

After the first primary round last month that saw more than 600,000 people take part, Marki-Zay and Dobrev were the final candidates battling it out.

Marki-Zay, 49, came third in the first round but persuaded the runner-up -- liberal Budapest mayor Gergely Karacsony, who had been the favourite -- to withdraw and endorse him in the run-off.

During the campaign he argued that only he can appeal to both leftist voters and conservatives tired of Orban's often divisive policies such as anti-immigration and anti-LGBTQ drives.

Ballot counting was already underway on Sunday afternoon in the race to challenge Viktor Orban Ballot counting was already underway on Sunday afternoon in the race to challenge Viktor Orban Photo: AFP / Attila KISBENEDEK

An economist and engineer who lived in the US and Canada for five years, Marki-Zay grabbed national attention in 2018 when he won the mayoralty in the small city of Hodmezovasarhely.

Although the southern city with a population of 44,000 had been a Fidesz stronghold for decades, he rallied cross-party support in what he called the blueprint for opposition success nationwide.

Despite having no party machinery or significant funding, Marki-Zay was also boosted during the primary race by support from younger voters open to his anti-elite and anti-corruption messages.

Dobrev, a vice president of the European Parliament since 2019 and vying to become Hungary's first woman prime minister, emphasised her greater experience and accused her rival of "unsuitability" for the top job.

But polls indicated the mother-of-three's weakness was her husband, former prime minister Ferenc Gyurcsany, who admitted lying in 2006 during a leaked private speech and has been relentlessly attacked by Orban ever since.

With the backing of DK, Hungary's largest opposition party headed by Gyurcsany, Dobrev, 49, won the primary's first round but fell short of an outright majority that would have won her the candidacy without a run-off.

After a bruising run-off with accusations thrown between the pair, Marki-Zay said Sunday during the vote count that "we want to reassure everyone that unity within the opposition is vital for both of us".

The primaries were called an "amazing success" by organisers, mobilising over 800,000 voters in total, almost 10 percent of the electorate in the 9.8 million population EU member.

"That's a lot of people even compared to countries with a long tradition of primaries, unlike Hungary where this has never happened before," Marta V. Naszaly, a Budapest district mayor who volunteered to count votes, told AFP Sunday.

"It gives legitimacy, the opposition will have candidates in next year's election who have a chance to change the government," she said.