Tens of thousands of people gathered in the Romanian capital of Bucharest on Friday to protest corruption and what critics of the government say is much-weakened rule of law in the European country. Hundreds wound up injured when police responded with tear gas and water cannons.

Romanian President Klaus Iohannis criticized the heavy-handed police response, calling it a “brutal intervention, strongly disproportionate to the actions of the majority of people.”

“The interior ministry must explain urgently the way it handled tonight’s events,” the centrist leader wrote in a statement posted to Facebook. He also said he requested the attorney general look into the legality of the riot police’s intervention.

Some police officers were also injured when protesters retaliated, with some reportedly throwing bottles. Reuters reported that at least 400 people sought medical attention and that the protests were organized in large part by Romanian expats working outside of the country.

At least 3 million Romanians live and work abroad, but local media reported that a number of them returned home to take part in this weekend’s demonstrations, which also took place in other parts of the country.

Agence France-Presse reported that another protest was expected to take place, with crowds already gathering in Bucharest in the early evening.

The demonstrations stem from some Romanians’ long-standing frustration with the ruling Social Democrats. Romania is one of the most corrupt countries in the European Union, according to Transparency International. The country’s anti-corruption agency has pursued a number of corruption cases, and in 2016 managed to prosecute 713 officials, according to the AP. Among them were a number of politicians, including a senator and 28 mayors.

But in July, Laura Codruta Kovesi, the leading anti-corruption prosecutor, was fired. Kovesi had support from the president, but the justice minister accused her of overstepping and it was ultimately Iohannis who was forced to fire her after a court ruling.

In June, a dozen countries, including the United States, warned against amending legislation “that would weaken the rule of law or Romania’s ability to fight crime or corruption.” Those changes made it through parliament but are now being challenged in court.