Thousands of protesters — including many expatriates returning home — hit the streets of Bucharest on Friday to demand the resignation of the government.
They are upset at alleged moves by the ruling Social Democrat Party (PSD) to weaken the judiciary and decriminalise some low-level corruption offences.
There was some violence: protesters tried to break through a security line while others threw bottles and rocks, reported Reuters.
Riot police used tear gas and water cannon to maintain order.
Video footage posted on social media show police beating non-violent protesters holding their hands up.
More than 400 people required medical assistance, the emergency intervention agency ISU said, including two riot police who got separated from their unit.
Tens of thousands staged peaceful protests in other Romanian cities.
Centrist Romanian President Klaus Iohannis condemned the police's disproportionate use of force.