Activists lobbed smoke bombs at police, who charged against demonstrators in the northern industrial city, which has been badly hit by a painful two-year recession. A few protesters were seen being detained.
Thousands of people also took part in a peaceful demonstration called by the main trade unions in Pordenone, near Venice, where the closure of a nearby washing machine plant owned by Sweden's Electrolux has put 1,300 jobs at risk.
"We always hear talk about cuts in Europe instead of investment in labour," Susanna Camusso, leader of Italy's biggest union, the CGIL, said at the rally.
There was also a major protest in Milan and May Day concerts in Rome and Taranto – a heavily polluted industrial city in southern Italy where thousands of jobs at the local steel plant are at risk.
Organisers said there were around 300,000 people at the Rome concert and 100,000 at the Taranto one.
Auto workers at the closed Fiat plant in Termini Imerese in Sicily also staged a sit-in outside the factory gates.
Although Italy's monthly unemployment rate went down slightly to 12.7 percent in March, from 13 percent in February, it was 0.7 of a percentage point higher than in March 2013, according to official data released Wednesday.
Italy began growing again in the fourth quarter of 2013 after its worst recession since World War II, but the government is forecasting the economy will grow by just 0.8 percent this year.
Speaking at an event in Rome, the Italian President Giorgio Napolitano said there was a "labour emergency" and called for "maximum reaction in terms of reforms and public policy".
He also paid tribute to several business owners and workers in Italy who have committed suicide in recent years because of the acute economic crisis.
At the Vatican, Pope Francis also had a May Day message.
In a tweet from his account, @pontifex, he wrote: "I ask everyone with political responsibility to remember two things: human dignity and the common good."