The hammer is close to everyone's heart in the host nation, who have won five Olympic men's golds in the event and seven of their 14 world silver and bronze.
Halasz, joint bronze-medallist in 2019 and fifth last year, launched his first throw and surpassed the automatic qualifying mark of 77 metres to the delight of the vocal morning crowd.
He will return to a bigger attendance and even louder roar on Sunday, Hungary's National Day.
Canada's Ethan Katzberg smashed his personal best by more than two metres with an enormous throw of 81.18 - a national record and the second-longest by anyone this year behind Pole Wojciech Nowicki's 81.92.
Nowicki also advanced on Saturday with 78.04 while compatriot Pawel Fajdek, who is seeking a sixth successive world title to match the record of pole vaulter Sergey Bubka, was due to compete in the second qualifying group later in the day.
