"We are not yet North Korea, but this amendment is extremely alarming because it removes constitutional control and checks over the Legislature," said Peter Hack, a professor of constitutional law.
Constitutional experts said the amendment, passed Monday, will allow the government to reintroduce measures rejected by the constitutional court in the last 18 months.
Those rejected measures include a ban on political advertising in political media and a law that would allow police to fine or ban homeless people, The New York Times said.
European Commission President Jose Manuel Barroso and Council of Europe head Thorbjorn Jagland issued a joint statement saying the changes raised "concerns with respect to the principle of the rule of law, EU law and Council of Europe standards."
Thousands of people protested the changes in Budapest Monday.
The amendment comes amid concerns related to government centralization and worries about a power grab by Prime Minister Viktor Orban, the Times said.