Dozens of people occupied the Budapest compound of Hungary's ruling conservative party for around 10 hours Thursday in protest against changes to the new constitution.
The parliament will vote Monday on a series of changes to the "Fundamental Law", which in 2012 replaced the previous constitution and has been criticised by some as restricting civil rights.
The mostly young protesters chanted "Constitution! Democracy! Rule of law!" during their demonstration inside the gated headquarters of the Fidesz party. One tried to enter the building but was stopped by police.
"A really worrying oppressive system is being built up here, like a dictatorship," Milan Rozsa, a 25-year-old protester, told AFP.
Opponents of the changes to the new constitution claim they accelerate an assault on democratic structures started by the Fidesz-led government in 2010.
But the government insists the package of changes -- the so-called "fourth amendment" -- is mostly technical.
It aims to reintroduce measures rendered void by the constitutional court in recent months, such as restricting election campaigning to state media or prohibiting sleeping on the streets.
The changes would also introduce new provisions, including a contract binding students who receive state grants to stay and work in Hungary after graduation, and a limitation to the powers of the constitutional court.
In a statement Fidesz said property was damaged during the protest, which ended late Thursday, and called it an act of "aggression."
It also claimed the unrest was the work of leading opposition figure and former prime minister Gordon Bajnai.
Bajnai's voter-mobilisation movement Together 2014 denied any involvement, claiming the regime was abusing its power and "keeps fixing the constitution according to momentary interests," it said in a statement.
The fourth amendment legislation has also led the Council of Europe, the European Parliament and the European Commission to express their concern.
In a letter Thursday to Council of Europe head Thorbjorn Jagland, Hungary's justice minister suggested holding consultations "should anything need to be clarified".
An opposition demonstration against the fourth amendment is scheduled to take place in front of parliament on Saturday.