Protesters and riot police clashed in Romania's capital as tens of thousands demonstrated against the government for decriminalising some official misconduct.
Critics at home and abroad called the move a major setback for the fight against corruption.
A handful of protesters threw firecrackers and smoke bombs at police guarding the main government offices, who responded with tear gas.
At least one person was detained and a newspaper kiosk was set on fire. Media reported that the violent protesters were football supporters and not anti-government demonstrators.
Emergency situations official Raed Arafat said two police officers and two demonstrators were treated at hospital for minor injuries. An unspecified number of other officers suffered light injuries.
It was the second consecutive night of protests in Bucharest against the government, whose emergency ordinance to decriminalise abuse in office went against warnings from prosecutors and the president.
The ordinance was published in the official government monitor at 3am on Wednesday.
The speed with which the centre-left government approved the proposal and the hour of its action alarmed critics.
The coalition government has been in office for less than a month and the ordinance benefits its allies and Romanian officials facing corruption charges.
There were protests in half a dozen cities around Romania, with people calling for the resignation of the government.
President Klaus Iohannis, who has limited powers and does not oversee the government, called the measure's adoption "a day of mourning for the rule of law".
In recent years, Romania has been touted as a regional leader for targeting the rich and the powerful in a crackdown on corruption. But the drive proved unpopular with politicians.
The leaders of the centre-left Social Democratic Party and the junior Alliance of Democratic Liberals, which form the current coalition government, both face corruption charges that bar them from serving as ministers.
Social Democrat chairman Liviu Dragnea was unable to become prime minister because in April last year he received a two-year suspended jail sentence for vote rigging.
On Tuesday, he went on trial for abuse of power while he was president of the Teleorman local council from 2006 to 2012. He denies wrongdoing.
Justice Minister Florin Iordache said the emergency ordinance will decriminalise cases of official misconduct in which the damages are valued at less than 200,000 lei (£37,000).
On Wednesday, Romania's Supreme Council of Magistrates unanimously agreed to take the emergency decree to the Constitutional Court, which is the last legal resort to stop the law.
Protests erupted in cities around the country after the decriminalisation plan was made public last month.
The chief anti-corruption prosecutor, Laura Codruta Kovesi, said it "will render the anti-corruption fight irrelevant".
The National Anticorruption Directorate has prosecuted 1,170 cases of abuse in office during the past three years with damages worth 1 billion euro (£850 million), just under one-third of all of its cases, she said.
The European Union criticised the Romanian government's move.
European Commission Vice President Maros Sefcovic said the EU is "following the latest developments in Romania with growing concern".
He said: "The fight against corruption needs to be advanced and not undone."