In his first strong political statement since his landslide election defeat, Orban late on Tuesday issued a "Statement of Resistance" on Facebook shortly after his Fidesz party said prosecutors had raided offices housing its internet servers.
Prosecutors told Reuters in an emailed response that the action was linked to an investigation into "misappropriation and other criminal offences" at the National Culture Fund under Orban's government.
Orban told reporters that prosecutors had taken servers containing a database and emails of party members, which he said was a politically motivated move to make it legally impossible for Fidesz to operate.
"The National Culture Fund issue was just a pretext for this," Orban said, adding that there had been no wrongdoing.
In his Facebook post, Orban said Hungary had "ceased to be a democratic state" after the new centre-right government of Prime Minister Peter Magyar passed a constitutional amendment ending the mandate of the president and the head of the constitutional court.
Tisza denies taking an authoritarian stance towards political opponents.
It has pledged to curb corruption, which it says was rampant under Orban's administration, part of Tisza's drive to bring Hungary back into the EU mainstream.
Orban himself faced heavy criticism during his rule over democratic standards in Hungary, and was in frequent conflict with the EU over issues such as the rule of law and minority rights.
Orban has denied the corruption allegations.
"The new political system created this way is illegitimate ... Fidesz will use all peaceful tools against abuse of power inside parliament and outside parliament," Orban said.
Magyar's Tisza party has a supermajority in parliament, which enables it to roll back changes introduced by Orban that Magyar says harmed democracy.
The constitutional amendment ended president Tamas Sulyok's term, citing society's "serious loss of confidence" in a leader elected in 2024 by MPs from Orban's Fidesz party.
Parliament will soon elect a new president.
Magyar's Tisza commands huge public support, with a poll by pollster Median in July showing 60 per cent support, against 18 per cent for Fidesz.
Since the election, Fidesz has been in disarray, with its parliamentary group leader resigning last week.
