A multiday primary election to find a candidate to challenge Hungarian Prime Minister Viktor Orban got under way on September 18 but was suspended due to a system crash.
Organizers blamed the early interruption of voting on a suspected cyberattack and said balloting would resume on September 20.
"In addition to the masses of voters seeking change, someone else was interested in the primary: a mass load of [messages of] unknown origin hit the background system of the primary election," the national primary election committee said in a statement.
The primaries are the first to seek a challenger to Orban and are taking place after a six-party alliance of leftist, liberal, and formerly far-right parties united under the common goal of pushing Orban out in elections next year.
They accuse Orban, who has clashed with Brussels over migration and rule-of-law issues, of corruption and creeping authoritarianism and hope the primary system brought in under Orban in 2012 will be their path to defeating his Fidesz party.
Antal Csardi, a candidate for the green LMP party, said the primary elections are "an innovation that was forced on us" by the election system, and are the only hope of seeing an anti-Fidesz candidate win.
"The opposition can only compete with Fidesz if they are in a single bloc,” Csardi told AFP. “We've learned that the hard way."
The primaries allow opposition voters to select single candidates to take on both Orban as well as Fidesz rivals in each of Hungary's 106 electoral districts.
More than 250 candidates are standing in the primaries, which are scheduled to run through September 26. Voting is taking place online and in-person. If necessary, a runoff for the prime minister candidate will be held from October 4-10.
Primary To Find Candidate To Oppose Hungary's Orban Gets Off To Rocky Start
The Hungarian parliament building in Budapest