The march was organised by Italy's main engineering union, the Fiom, whose leader Maurizio Landini is trying to unite the fragmented political left into what he calls a "social coalition" against Renzi.
He accuses the 40-year-old prime minister of moving his Democratic Party (PD) too far towards the political centre, abandoning it traditional roots.
"A new springtime begins today for Italy," Landini said, surrounded by supporters waving the Fiom's red flags. "We're ready to do battle knowing we have more support than the government does."
Renzi's so-called Jobs Act, key parts of which became law last month, makes it possible for firms to fire workers for business reasons with no risk of having to re-instate them if the dismissal is ruled illegal by a judge.
A company which wrongfully dismisses workers must pay them compensation but not give them their job back as before.
Renzi says the Jobs Act will help reduce record unemployment, which stands at around 42% among those below the age of 25.
The government is also offering temporary fiscal incentives for firms to hire workers on a permanent basis in an attempt to reduce the widespread use of temporary contracts which offer low pay and few rights.
Data presented by the labour ministry this week showed that new permanent contracts had risen by 79,000, or 35% in the first two months of the year compared with the same period of 2014, as firms take advantage of the incentives.
Renzi, who says he wants to "revolutionise" Italy with institutional and economic reforms, has so far shown scant regard to numerous strikes and protests against his year-old government by workers, students and opposition parties.
On Friday, he dismissed Landini's march, telling reporters: "Tomorrow there's another march against the government - no news." – Reuters, March 29, 2015.