More than 400 people were injured during an anti-government rally in Romania's capital, police say.
More than 50,000 people took part in in Friday's protest in Bucharest against what they described as entrenched corruption and low wages.
Several police officers were also hurt as some protesters threw bottles and police used tear gas and water cannon. A new protest is planned for Saturday.
President Klaus Iohannis said the police reaction was "disproportionate".
"I firmly condemn riot police's brutal intervention, strongly disproportionate to the actions of the majority of people," he said.
Protests were also held in several other Romanian cities.
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What happened in Bucharest on Friday?
The demonstration was staged in front of the government headquarters in the city centre.
The protesters were demanding the resignation of the government, objecting to the perceived efforts to weaken the judiciary by the governing Social Democrats (PSD).
The violence began when some in the crowd tried to break into the government building - but were held back by the police security cordon.
Other protesters were seen throwing paving slabs at the police, who responded with tear gas, pepper spray and water cannon.
Police said they had acted in a proportionate way, responding to the violent behaviour of hooligans in the crowd.
Many expat Romanians also returned to take part in the rally.
Ileana Anghel travelled all the way from her home in Spain with her husband to take part in the demonstration.
"We want to see modern roads and schools and above all to not have to pay bribes to the left and right," she told AFP.
Vlad, 60, flew back to Romania from New York, his home of 30 years.
"Corruption and embezzlement, which profit the ruling class, are what bothers me," Vlad told the same news agency.
According to the World Bank, up to a quarter of the Romanian population - between three and five million people - live and work abroad, sending back around $5bn (£3.9bn) to one of the EU's least developed countries.
What's the background to this crisis?
Protests have been building against the PSD for months.
In July, President Iohannis approved the dismissal of anti-corruption prosecutor Laura Codruta Kovesi, who had been leading corruption investigations into local and national politicians.
Mr Iohannis had been under extensive pressure from the PSD government to agree to her removal.
Around 150,000 gathered in Bucharest last year after the government passed a decree that could free those jailed on corruption charges.