Opposition MPs have chained themselves together outside the Hungarian parliament building, as part of a protest over new election and finance laws.
Campaigners claim both sets of legislation, passed later in the day, are a blow to democracy.
The European Commission had requested the laws be withdrawn, but the Hungarian Prime Minister refused.
“This is a very, very sad situation that we, as members of parliament, we were prepared to be in opposition in a way, to co-operate with every party in the parliament,” said Gabor Schering, one of the opposition MPs who took part in the demonstration.
“We are now forced to protect Hungarian democracy with ourselves, with our bodies.”
Former Socialist Prime Minister Ferenc Gyurcsany was briefly detained by police, along with several other lawmakers.
He said he did not know what action would now be taken against him.
Hungary’s parliament is dominated by Prime Minister Viktor Orban’s ruling centre-right Fidesz party.
Passing the disputed financial stability law could jeopardise talks with international lenders.
Critics say the new electoral law will change the country’s system in favour of Fidesz.