Protesters broke into Moldova's parliament after it approved a new government.
The vote was aimed at ending months of political deadlock but was boycotted by opposition parties.
Pavel Filip, from the main pro-European coalition, has been appointed prime minister.
The previous government of the former Soviet republic was dismissed by lawmakers in October amid a corruption scandal.
Thousands of people had gathered outside parliament in the capital, Chisinau, against the appointment of Mr Filip, a former technology minister.
The anti-government camp includes two pro-Moscow groups and one pro-European group.
A group of protesters broke police lines and forced their way into the building, local media reported.
Tear gas was reportedly used and policemen were seen wearing gas masks. It was not clear if there were any injured.
The protesters chanted "Cancel the vote!" and "Thieves", the AP news agency reported, and demanded early elections.
Protesters say that the new prime minister has close links to Vladimir Plahotniuc, one of the most powerful businessmen in Moldova.
President Nicolae Timofti last week refused to nominate Mr Plahotniuc for the post of prime minister and Mr Filip's opponents say he will now just be a proxy for vested business interests.
For his part, Mr Filip has pledged to form closer ties with the European Union.
Moldova is one of Europe's poorest countries and has been locked in political turmoil since the disappearance of some $1bn (£710m) from the banking system in 2014.
The missing money is equivalent to an eighth of the ex-Soviet republic's entire GDP.
The scandal caused a rapid fall in the value of the national currency, the leu, hitting Moldovans' living standards.