Addressing the rally, Peter Marki-Zay, who headed a united opposition but lost against Orban in April, said the nationalist premier's campaign promises had been "proven to be lies".
In his regular radio address on Friday, Orban defended the tax law change as "good and necessary".
Despite price caps on essentials, the central European country faces soaring inflation and a plunging local currency amid talks with Brussels over held-up EU funding.
Hungary, which largely depends on Russian oil and gas, declared a "state of danger" on Wednesday over the energy crisis sparked by the war in Ukraine.
Among the measures to tackle the problem, people who consume more than the average amount of energy will have to pay for it at the market price rather than the heavily subsidised state rate.
