The opposition candidate to Hungarian Prime Minister Viktor Orban, Peter Marki-Zay, pledged on Thursday to free his country from corruption and restore democracy with a new constitution if he is elected next year.

"If we manage to oust (Orban) in April... we will have to start from scratch," the 49-year-old conservative told a news conference in Brussels.

A new beginning is needed because "there is no democracy, no freedom of the press, no rule of law in Hungary," he said, pledging to "restore" these values.

"We are talking about a change of regime, not just a change of government," he said. "It's like Nazi crimes cannot be prosecuted under Nazi laws," he said.

Marki-Zay, a practising Catholic and father of seven, became the opposition candidate in a primary organised by a six-party opposition alliance last year.

Peter Marki-Zay: 'We will have to start from scratch'
Peter Marki-Zay: 'We will have to start from scratch' AFP / JOHN THYS

The unprecedented primary contest was an effort to combat the mainly first-past-the-post election system that favours Orban and his ruling Fidesz party.

While experts predict a close battle against Orban, the opposition leader acknowledged that it would be "extremely difficult" to defeat the nationalist leader, in power from 1998 to 2002 and continuously since 2010.

The elections will be "neither democratic, nor free, nor fair", Marki-Zay warned.

"But our chance (of beating Orban) has never been greater in a decade," added Marki-Zay, who met several EU commissioners in Brussels, including the vice-president responsible for values and transparency, Vera Jourova.

Asked about the risk of unrest similar to the invasion of the Capitol in the United States if the opposition won, Marki-Zay said he "cannot imagine masses taking to the streets to defend the rights of criminals".