In Bucharest protestors gathered spontaneously in front of the seat of government, chanting "thieves" and calling for the resignation of the social democrat government, which has been in power for less than a month, AFP reporters saw.
Protests also took place in several other cities, including Cluj, Sibiu, Timisoara and Iasi, according to Romanian media reports.
They erupted immediately after the announcement of the emergency decree which could allow many politicians to escape criminal prosecution.
Earlier this month the government of Prime Minister Sorin Grindeanu published the emergency decrees, sparking angry protests across the country and criticism from Romania's centre-right President Klaus Iohannis, who was elected in 2014 on an anti-graft platform.
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"It is a day of mourning for the rule of law, which has received a hard blow from the opponents of justice and in the fight against corruption," the head of state said in a statement yesterday.
"This government has chosen to pass its ordinances secretly because it is afraid," said Martina, an international relations student at the demonstration in Bucharest, fearing a "backtrack" in the fight against corruption.
"Nothing gives them the legitimacy to do that ... It's a return to the period of 15 years ago," echoed Cristian Clot, an IT employee.
The gatherings dispersed without incident.
The legal changes decriminalise several offences and makes abuse of power punishable by incarceration only if it results in a monetary loss of more than 44,000 euros (USD 47,500).
Prosecutors have put the amount of money involved in his case at 24,000 euros, well below the new threshold.