League leader Matteo Salvini accused Mr Renzi of substituting the country's interests to those of the EU.
He also criticised the government's record in dealing with Romanian truck drivers, tax, banks and big business. A counter-demonstration against Mr Salvini was also held in Rome. Opinion polls suggest that Mr Salvini is rapidly gaining in popularity. They show him as being second only to Mr Renzi, prompting some to dub him as "the other Matteo".
The Northern League was once a strong ally of former Prime Minister Silvio Berlusconi, but it has sought to find new allies as he struggles to shake off a tax fraud conviction that forced him out of parliament.
Mr Salvini's fiery rhetoric against the European Union, immigration and austerity politics had led to comparisons being drawn between him and French National Front leader Marine Le Pen.
The counter-demonstration staged by an alliance of leftist parties, anti-racism campaigners and gay rights groups was held only a few hundred metres from the Northern League rally.
Many protested under the banner "Never with Salvini".
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"The problem isn't Renzi, Renzi is a pawn, Renzi is a dumb slave, at the disposal of some nameless person who wants to control all our lives from Brussels," Mr Salvini told the rally at the Piazza del Popolo.
He told his supporters that the prime minister was the "foolish servant" of Brussels.
Mr Salvini spoke of a "different Europe, where banks count for less, and citizens and small businessmen count for more".
"I want to change Italy. I want the Italian economy to be able to move forward again, something that is obstructed by Brussels and mad European policies," he said, describing the government's immigration policies as "a disaster".
The two protests come just over two months ahead of regional elections in May.
Analysts say Mr Salvini could emulate other nationalist European leaders and capitalise on growing resentment against immigration, especially after recent attacks by Islamic extremists and the influx of migrants brought to Italy by Libyan-based smugglers.