The liberal mayor of Hungary's capital announced Saturday that he will enter an upcoming primary race that will choose a candidate to face nationalist Prime Minister Viktor Orbán in closely watched elections next year.

In a video posted to Facebook, Budapest mayor Gergely Karácsony said he would accept the nomination of his party and run in a two-round primary in September and October as part of a six-party opposition coalition that seeks to unseat the governing Fidesz party.

"I made this decision because I feel that my homeland is in big trouble," Karácsony said, adding that he believes the biggest problem facing Hungary is polarisation dividing the country's citizens. "I would like to serve the purpose of reuniting Hungary," he said.

Karácsony, 45, was elected mayor of Budapest in 2019 as part of an effort by six opposition parties to join forces against Orbán's right-wing Fidesz, which has firmly governed Hungary with a two-thirds parliamentary majority since 2010.

Those municipal elections led to major losses for Fidesz in many of Hungary's cities, and the same six parties plan a repeat of their unity strategy in national elections next spring, expected to be the most competitive in more than a decade.

The opposition coalition contains Greens, Socialists, and centrist liberals, but has also found common cause with the right-wing Jobbik party, which has since 2018 sought to break ties with its radical, antisemitic past and shift to become a centre-right people's party.

Fidesz has criticised its liberal opponents for collaborating with Jobbik.

Critics of Hungary's government accuse Orbán of clamping down on media and judicial freedom, unilaterally authoring a new constitution and making unfair changes to election laws. But Orbán asserts that Hungary is pursuing an innovative experiment in what he calls "illiberal democracy," based on Christian conservatism and a firm rejection of immigration.

An OSCE election monitoring delegation in 2018 found that "intimidating and xenophobic rhetoric, media bias and opaque campaign financing" had made elections that year unfair, but still characterised the voting process as free.

Hungary's opposition parties argue that electoral changes have meant that coordinating their efforts into a single bloc against Fidesz is the only means to unseat the party. Recent polling finds that Fidesz and the opposition coalition are neck and neck in voter support.

In his video on Saturday, Karácsony alluded to allegations of corruption against Fidesz, and vowed to serve the "99 per cent" if he is elected prime minister, invoking a left-wing populist slogan used by the Occupy movement nearly a decade ago.