Budapest, Hungary (CNN) -- Tens of thousands of people protested against Hungary's new constitution in Budapest Monday night, demanding that Prime Minister Viktor Orban resign.
Crowds outside the city's opera house called the prime minister "Viktator" -- a pun on "dictator" -- as Orban and other dignitaries attended a gala inside.
Lasting nearly five hours, the protest was organized by opposition parties and civil society groups who say the new constitution is anti-democratic.
American Secretary of State Hillary Clinton expressed similar concerns last year, pushing Orban to commit to "the independence of the judiciary, a free press, and governmental transparency."
Orban and other officials left the gala celebrating the new constitution through back doors to avoid the demonstration.
The new constitution, which took effect on January 1, omits "republic" from Hungary's official name, and includes several paragraphs which Amnesty International says violate international human rights.
The human rights organization has been critical of sections that define life as beginning at conception and marriage as being between a man and a woman.
It also expressed concerns about the possibility of life imprisonment without parole, and failure to forbid discrimination on the grounds of sexual orientation.
Monday's demonstration was preceded by a series of protests in 2011 against laws introduced by Hungary's populist government.
Critics have said the new media law is restrictive and the new electoral law favors Orban's ruling party FIDESZ.
They have also taken aim at a law criminalizing homelessness.
European Commission President Jose Manuel Barroso wrote to Orban in December requesting the withdrawal of two recent bills related to the country's financial stability and the central bank. Orban rejected the requests.