Italian police have clashed with demonstrators protesting against the Milan Expo, which the government had been hoping would lift a gloomy national mood.
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.
With 10 million tickets already sold, officials are counting on about 20 million people attending, and hope overall revenues will top $10.75bn, 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.