The march against wars and rising living costs in Italy began peacefully on Saturday but masked protesters threw stones, paving blocks and other objects at riot police and set fire to barricades made from bar tables.
Images of the aftermath showed a severely damaged carabinieri car.
Police used water cannons and tear gas to disperse the crowds.
At least two police officers were injured and about 50 demonstrators, including many minors, were detained, the LaPresse news agency reported.
Italian Premier Giorgia Meloni condemned the violence in a social media post.
"Demonstrating is a right. Setting our cities ablaze is not," she wrote.
"Hooligans who exploit demonstrations to sow chaos, attack those responsible for ensuring security and hold entire cities hostage," she wrote on x.
Milan's centre-left mayor, Giuseppe Sala, said "the reasons for the protests were understandable. The violence against law enforcement and serious damage to the city are not justifiable".
The unrest coincided with nationwide demonstrations in neighbouring France against rising living costs and in support of high school students' grievances.
French police used tear gas and water cannons to disperse violent protesters who joined the rallies.
