Grassroots union Unione Sindacale di Base called a 24-hour general strike, with protests planned in more than 40 cities, Reuters reported.
The demonstrations, which came after clashes earlier this month in which protesters fought with police, looted shops and vandalized property, were largely peaceful, although tensions flared in some areas.
In Turin, police used a water cannon to push back students and left-wing activists who approached security cordons protecting the prefecture building.
A large march passed near the Colosseum, in Rome, where protesters said they planned to set up tents and remain over the weekend. Some rubbish bins were placed across streets and set on fire. In the evening, a large group tried to march towards parliament, prompting police to use water cannon to disperse the crowd, Italian news agency ANSA reported. Eight people were detained after repeatedly trying to break through police barricades, it said.
In Milan, protesters marched onto the city's ring road before ending the march in front of the Politecnico di Milano University. Demonstrators carried flares and set fire to objects.
Around 15,000 port workers, students and other demonstrators entered the port of Genoa, joining similar protests in Europe and elsewhere against the use of ports for arms shipments and highlighting the deaths of seafarers in conflicts from the Gulf to the Black Sea.
A wave of protests has swept across European countries including France, Spain, Belgium and Albania, driven by grievances including the cost of living.