BUCHAREST: Pressure intensified on Romania's government on Monday after half a million people joined the biggest nationwide protests in almost three decades, with many calling for it to quit.

Demonstrators vowed no let-up as a no-confidence opposition motion was set for its first reading in parliament, although the government's majority means it is unlikely to pass.

The sixth straight evening of demonstrations on Sunday came despite the left-wing government scrapping the measure that started it all: an emergency decree weakening anti-corruption laws.

Brandishing banners, waving flags and blowing whistles and vuvuzela horns, protesters said they believed the government was still not to be trusted, with many chanting "Resign! Resign!"

"They are corrupt. We want justice... The government will still try something (with the decree)," Emma, 24, one of around 250,000 protesters that media estimated were in downtown Bucharest, told AFP.

"They are liars and bad people," her friend Nicole, 25, shouted above the racket at Victory Square, the epicentre. "The government has to fall... We are going to come back here every night."

Despite the protests, which are the biggest since people power brought down dictator Nicolae Ceausescu and the communist system in 1989, the government remained defiant.

Prime Minister Sorin Grindeanu said his Social Democrat party (PSD), which won a resounding election victory only in December, has "a responsibility to the people who voted for us" and would stay.

PSD head Liviu Dragnea -- already barred from office for a voter fraud conviction and currently on trial for alleged abuse of power, a charge he denies -- said he believed the protests were being instigated by unspecified powerful forces.

"Who is organising this? Hard to say but I hope that the state institutions have this information," Dragnea said in a television interview.

"I reproach myself for not having understood that this is a much better organised plan than a simple spontaneous movement."

- Get out of jail -

The government decree, which had been scheduled to enter into force on February 10, was to make abuse of power a crime only punishable by jail if the sums involved exceeded 200,000 lei (44,000 euros, $47,500).

One Monopoly-inspired placard on Sunday read: "Do not collect 200,000. Go straight to jail."

The government still also aims, via a separate decree to be reviewed by parliament, to free some 2,500 people from prison serving sentences of less than five years.

Grindeanu has argued the measures were meant to bring penal law into line with the constitution in the EU member state and reduce overcrowding in prisons.

But critics see the moves as a brazenly transparent attempt to let off the many officials and lawmakers who have been ensnared in a major anti-corruption drive in recent years.

That push, which won Romania kudos abroad, has seen almost 2,000 people convicted for abuse of power and a serving prime minister and a string of ministers and lawmakers go on trial.

Justice Minister Florin Iordache said that a bill taking into account all the grievances about the decree would be published later Monday and be debated with opposition parties and civil society groups.

But demonstrators said they would continue to protest.

"If we hit the streets again this evening (the government) won't be able to resist. It's today or never," wrote user Galbus on Facebook group "Corruption Kills".

Political analyst Cristian Tudor Popescu compared the government to a "thief who tries to be pardoned if he brings back what he has stolen".