New Delhi, Oct. 23 -- Hungary's two main political parties will mark a national holiday on Thursday by holding rival demonstrations that will likely bring hundreds of thousands to the streets and kick off a campaign for elections next year.

With the ballot approaching in April, long-serving Prime Minister Viktor Orban - trailing in most polls to his main political rival, Peter Magyar - hopes to use Thursday's show of force to reinvigorate his base that has had its confidence shaken by a series of political scandals and years of high prices and economic stagnation.

Magyar, a onetime insider within Orban's Fidesz party who erupted onto Hungary's political scene last year, aims to channel a growing number of dissatisfied voters into what w...