Wielding red flags and placards, they urged the centre-left prime minister, Enrico Letta, to scrap austerity measures and focus on job creation.
Public trust in his fragile coalition with the centre-right is dropping, opinion polls suggest.
Italy is experiencing its longest recession in more than 40 years.
National debt is now about 127% of annual economic output, second only to Greece in the eurozone.
Unemployment is at a record high of 11.5% – 38% for the under-25s.
Before taking office, Enrico Letta vowed to make job creation his priority, but critics are unhappy that he has focused on property tax reform.
The issues of social justice and poverty came up when German Chancellor Angela Merkel had talks with the new Pope, Francis, at the Vatican on Saturday.
Organized by the metalworkers’ union FIOM and the CGIL union, Saturday’s peaceful march and rally drew supporters from across Italy. The turnout was unclear but 50,000 people had been expected to attend.
“We ask the government to change [former Prime Minister Mario] Monti’s and [former Prime Minister Mario Silvio] Berlusconi’s politics,” said Maurizio Landini, leader of the FIOM.
“If they don’t change, as the country asked for with its vote, we are going nowhere.”
One of the protesters, Enzo Bernardis, told Reuters news agency: “We hope that this government will finally start listening to us because we are losing our patience.”
Soon after being appointed, Enrico Letta met other eurozone leaders to convey growing public unrest over austerity measures in Italy.
But the new prime minister has to maintain a delicate balance between the policies of his own supporters and those of the centre-right, led by Silvio Berlusconi.
Italy’s coalition was only formed after two months of post-election deadlock.
Among the demonstrators in Rome were radical leftists.
A controversial poster depicted Chancellor Angela Merkel, who is seen as typifying austerity, in mock-Nazi uniform.
On Thursday, Pope Francis said in a speech that the global economic crisis had made life worse for millions in rich and poor countries.
Speaking after her private meeting with the pontiff, Angela Merkel told reporters: “Crises have blown up because the rules of the social market have not been observed…
“It is true that economies are there to serve people and that has by no means always been the case in recent years.”
Angela Merkel said she and Pope Francis had spoken mainly about globalization, the European Union and the role of Europe in the world.
“Pope Francis made it clear that we need a strong, fair Europe and I found the message very encouraging,” she added.
While she is not a Catholic herself, Angela Merkel, the daughter of a Lutheran minister, leads a party with a strong Catholic component.
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