Italian police have clashed with demonstrators protesting against the Milan Expo, which the government had been hoping would lift a gloomy national mood, Al Jazeera reported.
Police fired tear gas at protesters, many of whom see the event as a symbol of capitalism, on Friday as clouds of smoke from burning cars filled parts of the centre of Milan.
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 where Prime Minister Matteo Renzi hailed the start of a six-month-long showpiece of culture and technology, focused on the theme of sustainable food production.
The fair, which follows the 2010 Expo in Shanghai, mobilised a diffuse range of left-wing protesters, from anti-globalisation and environmentalist activists to students and anti-austerity campaigners, who see it as a symbol of waste and corruption.
Thousands of police had been deployed to counter the threat of violence before the event, which Renzi has been counting on to reinforce fragile signs of economic recovery after years of stagnation and recession.
"Today it is as though Italy is embracing the world," he said at the opening, which featured a fly-past by jets trailing the colours 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 centre of Milan was transformed into a battle ground, with sirens and periodic bangs from flash bombs and firecrackers ringing out against the shouts of protesters.