More than 200,000 Romanians across the country protested for a third straight day as the prime minister refused to squash a decree that decriminalises a range of corruption offences.

Thursday's protests were among the biggest rallies in Romania since the end of communism in the late 1980s.

Many shouted "Thieves!" and called on the government to resign.

Earlier, after a meeting in Bucharest of his ruling Social Democrats, Prime Minister Sorin Grindeanu said: "We took a decision in the government and we are going to press ahead."

His defiant remarks came after the country's business and trade minister resigned over the decree.

Passed late on Tuesday, the decree decriminalises certain corruption offences and makes abuse of power punishable by jail only if the sums involved exceed 44,000 euros ($47,500).

Florin Jianu, minister of business, trade and entrepreneurship, called his resignation an "ethical" decision.

"Not for my professional honesty, my conscience is clean on that front, but for my child," he wrote on Facebook.

"Am I going to tell him his father was a coward and supported actions he does not believe in, or that he chose to walk away from a story that isn't his?"

IN PICTURES: Protests surge as Romania decriminalises corruption

According to local media, more than 200,000 people continued the protest on Thursday in the capital and dozens of cities across the European Union state, where a crackdown on corruption over the past four years has uncovered myriad conflicts of interest and a pattern of local officials handing out state-funded contracts in exchange for bribes.

A day earlier, some 250,000 people protested, with the majority taking to Bucharest's streets to decry the decree.

The recent decree was adopted barely a month since the Social Democrat-led government of Grindeanu took power.

OPINION: Romania - Keep the corrupt in jail, where they belong

Romania's top judicial watchdog has filed a constitutional court challenge to the decree, which will take effect in 10 days unless blocked by a court ruling.

Grindeanu sent a letter on Wednesday to European Commission head Jean-Claude Juncker explaining why his cabinet chose to pass the decree and a draft bill granting prison pardons for several offences.

The government said it was designed to ease prison overcrowding and bring the criminal code into line with recent constitutional court rulings.

READ MORE: Bucharest - Thousands protest decriminalising corruption

Juncker on Wednesday said he was watching developments with "great concern", warning that the fight against corruption in Romania "needs to be advanced, not undone".

Six western states including Germany and the US have also criticised the government.

"The Social Democrats are testing how far they can go," Laura Stefan, an anti-corruption legal expert said. "If we accept they can approve emergency decrees untransparently, then tomorrow they will adopt others and so on. We will wake up without institutions overnight."