More than 400 needed medical attention after police used tear gas and batons to quell demonstrations by a huge crowd said to be swelled by thousands who had returned from abroad to take part.
Local media estimated between 30,000 and 50,000 gathered in the capital Bucharest but protests were also organised as cities across the country.
They were angry at what they say is entrenched corruption, low wages and attempts by the left-wing Social Democrat government (PSD) to weaken the judiciary.
Video footage on social media showed police beating non-violent protesters who had been putting their hands up.
In other spots in Bucharest, groups tried to force their way through police lines and threw rocks and bottles.
As the protests continued late into the night, riot officers used water cannon as they tried to break up the gathering.
Centrist Romanian president Klaus Iohannis condemned the police’s heavy handed use of force.
He wrote on Facebook: “I firmly condemn riot police’s brutal intervention, strongly disproportionate to the actions of the majority of people in the square.
“The interior ministry must explain urgently the way it handled tonight’s events.”
One of the thousands of expat Romanian who had returned for the demonstration, Italian resident and truck driver Daniel Ostafi, said: “I left to give my children a better life, which was not possible here then.
“Unfortunately, it is still not possible, the … people who govern us are not qualified and they are corrupt.”
Mihai Podut, 27, a construction worker who left in 2014, first for France and later Germany, added: “Almost all of the public sector is malfunctioning, it must be changed completely and replaced with capable people.
“I would ask our ruling politicians to switch places with us, work the way we do and see what that is like.”
Since the PSD won power in 2016, the government has proposed new laws that critics say weaken the country’s fight against corruption.
Around three to five million Romanians are thought to live abroad, some in the UK but most in Italy, Spain and Germany.
Some say they left because of corruption, low wages and lack of opportunities.