Romania's capital Bucharest was bracing for another big demonstration on Saturday, only a day after more than 450 people were hurt and around 30 arrested in a huge anti-corruption protest.

Hundreds of people -- some carrying Romanian flags -- started gathering around a central square in Bucharest for a second day of protest, with the crowd expected to grow as the evening progresses.

Police had used water cannon and tear gas to disperse the protesters as tens of thousands rallied on Friday to call on the left-wing government to resign.

Many demonstrators needed treatment after inhaling pepper spray and tear gas, while others suffered blows, hospital sources said.

Around 30 police were also injured, 11 of whom were taken to hospital.

Police rejected criticism from the center-right opposition that its officers had used excessive force, saying its response to violence by dozens of protesters had been "gradual and proportionate".

Austrian Chancellor Sebastian Kurz, whose country currently holds the EU rotating presidency, criticized the clashes, which also saw a cameraman for Austria's public broadcaster hurt.

"We strongly condemn the violent clashes in Bucharest where numerous demonstrators and journalists were injured. We expect full explanations," he said on Twitter.

"Freedom of expression and, related to that, freedom of the press are basic freedoms of the EU, which we clearly recognize and which must be unconditionally protected."

Local media said up to 80,000 people had taken to the streets, among them many Romanian expatriates who returned home to show their anger at the graft in one of the EU's most corruption-plagued member states.

About a thousand officers in riot gear intervened to clear the crowds assembled in a central square outside the main government building after some protesters tried to break through the police cordon.

Romania's center-right President Klaus Iohannis, a critic of the government, said he "strongly condemned the brutal intervention of the police, which was disproportionate to the attitude of most demonstrators" but added that "any form of violence is unacceptable".

Prime Minister Viorica Dancila, in turn, accused Iohannis of "inciting the population against the authorities" and "using the dramatic effects of the events".

In a controversial move last month, Romania sacked top anti-graft prosecutor Laura Codruta Kovesi -- considered a symbol of the country's fight against corruption.

With Kovesi at the helm, the Anti-Corruption Prosecutor's Office (DNA) had led a crackdown on corruption among local and national elected officials, earning the enmity of many in Romania's political class and prompting critics to accuse it of abuse of power.

Before Kovesi's sacking, thousands of protestors took to the streets in support of her. There have also been long-running waves of protests against judicial reforms -- at their peak drawing an estimated half a million people nationwide in February 2017.

The demonstrations thus far had been largely peaceful.