Marki-Zay defeated leftist Klara Dobrev, who pledged to support him at the head of an alliance of six opposition parties that, in the 2022 parliamentary election, will bid to oust Orban after more than a decade in power.

"We can only win together," Marki-Zay told a crowd of cheering supporters, accompanied by his wife and seven children. "No one can break the unity of the opposition."

"This was a battle, but we have to win the war as well," he said, referring to next year's ballot.

He pledged to bridge divisions in society, clamp down on corruption, and fight in what he called an "uneven playing field" with most media owned by businessmen close to Orban's Fidesz party.

"We want a constitution where checks and balances prevent someone from grabbing power," he said.

With 74% of votes counted in the primary and final results due later on Sunday, Marki-Zay had 57% of votes to the Democratic Coalition's Dobrev's 43%.

In a statement Fidesz said Marki-Zay had signed a pact with the political left to help them return to power, and raise taxes, labelling him a "career leftist."

COALITION OF 'THE CLEAN'

For the first time since he came to power in 2010, Orban will next year face a united front of opposition parties that also includes the Socialists, liberals and the formerly far-right, now centre-right, Jobbik.

Opinion polls show Fidesz and the opposition alliance running neck-and-neck.

Marki-Zay, 49, has portrayed himself as a palatable choice for both left-wing and conservative voters, with his family-man image and Christian faith likely to appeal to swathes of undecided voters.

Holding degrees in economics, marketing and engineering, he rose to prominence when he won a mayoral contest in 2018 in his southern hometown, Hodmezovasarhely, a Fidesz party stronghold.

Marki-Zay has campaigned on leading a coalition of "the clean", promising to root out corruption, and he said he and Dobrev had agreed ahead of the race that keeping the opposition united was vital.

Both are looking to dismantle what they describe as Orban's "illiberal state", including its ideological foundations, Hungary's constitution and a raft of new laws that critics say have helped Orban cement his grip on power.

While Orban thrives on conflict and has had a series of disputes with the European Union, both Marki-Zay and Dobrev are looking to improve relations with Brussels. They are also in favour of Hungary adopting the euro in the foreseeable future.