Police fired tear gas at demonstrators on Friday as protests overshadowed the start of the Milan Expo, a global fair the government had been hoped would help to put a new face on Italy after years of economic decline.
Thick clouds of smoke from burning cars filled parts of central Milan, where groups of protesters, their faces masked against the fumes, threw stones and faced off against lines of police in riot gear.
The confrontation came hours after a glitzy opening ceremony at the Expo site, with Prime Minister Mateo Renzi hailing the start of a six-month-long showpiece of culture and technology that focused on the theme of sustainable food production.
The fair, which follows the 2010 Expo in Shanghai, mobilized a range of left-wing protesters, from anti-globalization and environmentalist activists to students and anti-austerity campaigners, who see it as a symbol of waste and corruption.
Thousands of police were deployed to counter the threat of violence before the event, which Renzi has been counting on to reinforce fragile signs of recovery after years of recession that have hit young people especially hard.
"Today it is as though Italy is embracing the world," he said at the opening, which featured a flight of jets trailing the colors of the Italian flag. "All you experts who kept saying, 'We'll never do it' — this is your answer," he said. "I like to think that tomorrow begins today."
Instead, the elegant center of Milan was transformed into a battleground, with sirens and periodic bangs from flash bombs and firecrackers ringing out against the shouts of protesters.
The main part of the demonstration was halted by police vans blockading the street, but breakaway groups of demonstrators fought running battles in the rain that left streets littered with makeshift barricades and debris.
Police detained a number of demonstrators. At one point, officers who put a woman in handcuffs were surrounded by an angry crowd demanding that she be released.
There was no immediate official estimate of the damage, which included smashed windows and burned-out cars.
How far the protests will tarnish the image of the Expo remains to be seen. They are likely to come as a severe embarrassment for the government, after the optimism of the opening ceremony.
The pavilion of United Arab Emirates is seen at Expo 2015 in Milan, Italy, May 1, 2015.
Some 10 million tickets are already sold, and officials are counting on some 20 million people attending. They hope overall revenues will top 10 billion euros ($10.75 billion), half from foreign visitors drawn to Milan.
But the event had already been tarnished by a corruption investigation that saw several top officials arrested, and by cost overruns and construction hold-ups. Large parts of the site were not ready for opening day.
Pope Francis, who spoke via a televised link-up to the opening ceremony, referred to the irony of a global mega-spectacle dependent on corporate sponsorship deals being devoted to sustainable development and feeding the poor.
"In certain ways, the Expo itself is part of this paradox of abundance. It obeys the culture of waste and does not contribute to a model of equitable and sustainable development," he said.
The real protagonists of the event should be "the faces of the men and women who are hungry, who fall ill and even die because of an insufficient or harmful diet," he said.
The Italian pavilion is seen at Expo 2015 in Milan, Italy, May 1, 2015.
Cultural events, futuristic architecture, a "supermarket of the future" and dozens of restaurants fill the site, which, critics note, required more than 1 million square meters (250 acres) of farmland on the outskirts of Milan to be concreted over.
In all, more than 140 countries are taking part. China, an increasing presence in Italy after a string of high-profile business acquisitions, is particularly well-represented.