The protesters are demanding that parliament be dissolved
Several thousand people have been protesting in the Hungarian capital Budapest, over the appointment of a new prime minister, Gordon Bajnai.
Police said protesters threw objects and tried to break through fencing surrounding the parliament building.
There were reports that several police and protesters were injured in clashes.
Mr Bajnai replaces Ferenc Gyurcsany, who announced his decision to resign in March, saying he considered himself a hindrance to further reforms.
Mr Bajnai, a non-aligned figure who had been serving as the economy minister, was sworn in by parliament late on Tuesday.
He had been elected by 204 of the 386 members of parliament and has said his "only goal is that a year from now, Hungary will be in better shape".
But the anti-government demonstrators in Budapest are demanding that parliament be dissolved and an early election called.
A group of protesters on motorbikes drove into a police cordon and police used tear gas as hundreds of people broke through metal fences.
They "tried to break through the fences around parliament and threw different kinds of objects at policemen", said one police officer.
'Willingness to work'
Mr Bajnai has promised to implement tough austerity measures to tackle the country's grave economic problems.
Mr Bajnai has said he does not intend to stand in the next general election
He has said he will call elections if he cannot secure parliamentary support for his policies.
"To restart the economy, we need to cut the tax burden on employment, increase the willingness to work and support work-intensive economic fields with high added-value," he told parliament.
The opposition Fidesz party, which abstained from the vote and has called for elections, said the appointment of Mr Bajnai was an "extension of the mandate of a failed prime minister".
Party leader Tibor Navrasics said the government was afraid to go before the voters and would "do anything, any power-maintaining manoeuvre, just to extend their reign of power".
As expected, Mr Bajnai has nominated several political outsiders for key cabinet posts, including Peter Oszko, the head of a consulting firm, as finance minister.
Mr Bajnai said he expected Mr Oszko to "keep the budget in balance, pay attention to the frugal operation of the state, and at the same time create the necessary conditions for growth by drawing up a new and simplified tax code".
Hungary has been badly hit by the global economic crisis, and needed a $25.1bn (£16.9bn) IMF-led rescue package last November to avoid collapse.
Analysts say Hungary is heavily dependent on the loan to finance its massive state debt.
Last month, Mr Bajnai said quick and credible reforms could restore investor confidence and put Hungary back on track towards the euro.