Péter Magyar, a former government insider seen by some observers as the first serious threat in years to Orbán, staged the rally attended by around 10,000 people in the eastern town of Debrecen (225 km east of Budapest), a stronghold of the ruling Fidesz party.

Fidesz candidates have carried the town since the late 1990s with the ruling party winning about 60% of the vote at the last election compared with around a third for opposition parties.

Addressing flag-waving supporters, Magyar took aim at one of Orbán's core policies, criticizing what he called the extremely low level of child support allowances in Hungary.

"If you tell this story anywhere in Europe, no one is going to believe you," Magyar said.