Protesters clashed with police in the Hungarian capital Budapest overnight during ongoing demonstrations against new legislation that has been dubbed the "slave law".
The change allows employers to ask staff to work up to 400 extra hours per year of overtime instead of 250. It could amount to another eight hours a week for some workers, equivalent to an extra working day.
An amendment that has also enraged unions is that employers will be able to delay payments for the extra hours for up to three years. Previously it was one year.
The legislation was pushed through by Prime Minister Viktor Orban's ruling Fidesz party earlier in the day by way of their large majority.
Opposition MP's sought to block the vote and some held up a banner reading "Thank you Fidesz" with "Year of the Families" crossed out and "Year of the Slaves" written instead. The stated aim of Orban's administration is to help families.
Protesters shouting "Traitors, traitors" and "Orban go to hell" faced off against hundreds of police who stood on the steps of the parliament building.
A Reuters photographer said the crowd then moved towards a side gate, where some threw objects at police, who responded with pepper spray.
Viktor Orban's government has built a system critics see as autocratic, affecting businesses, academia, the courts and the media, but he has rarely angered different domestic voter groups at the same time.
Some people have also been angered by a new law passed on the same day to set up courts overseen directly by the justice minister.
The administrative courts will take over cases about government business such as taxation and elections currently handled in the main legal system. The government said the courts would be presided over by independent judges who would be able to handle cases more efficiently.
Critics say it will allow political interference in judicial matters and further undermine the rule of law.
In a statement, the rights group the Helsinki Committee said the law "is a serious threat to the rule of law in Hungary and runs counter to values Hungary signed up to when it joined the European Union."