The May 1 opening of the Milan Expo 2015 architectural show has been marred by violent clashes between a group of protesters and Italian riot police. Reuters reports that “thick clouds of smoke from burning cars filled parts of central Milan” as masked individuals threw rocks and petrol bombs, and law enforcement retaliated with teargas. The wider, student-organized No Expo protests—which have otherwise been nonviolent—are speaking out against the immense cost of the temporary Expo at a time when Italy is facing economic hardship.

The Milan Expo, which features 140 pavilions commissioned from a range of countries and companies, has faced significant criticism in the months leading up to its opening. According to Dezeen, the Italian Government has spent €1.3 billion ($1.5 billion) on the Expo, much to the anger of anti-austerity campaigners. The organization of the Expo has been marked by a series of embarrassments, including the arrest of the commissioner of the Italian pavilion amid allegations of corruption and bid-rigging.

Related: Bad news for Milan Expo 2015 as reports show pavilion construction incomplete

Last month Wolfgang Buttress, the designer behind the UK pavilion, said that “a number of pavilions ‘haven’t a hope in hell’ of completing,” ahead of the May 1 opening, and the Italian government awarded an additional €3 million contract for camouflage screens to hide ongoing construction. Expo masterplanner Jacques Herzog criticized the entire event as a “vanity fair,” and even the Pope has weighed in, saying, “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.”

Moreover, environmental activists meanwhile have expressed concern that the temporary nature of the Expo, with many pavilions slated for destruction at the end of the six month show, runs counter to the Expo’s theme of “Feeding the Planet, Energy for Life.”

So it’s perhaps of little surprise that the graffiti tag “NO EXPO” has been appearing around Milan in recent months, and that activists banded together to protest the opening of the show. Thousands of police were reportedly mobilized around the opening, at which Prime Minister Matteo Renzi spoke, as fears of violence mounted. While the majority of protesters remained non-violent, Reuters reports that “the elegant centre of Milan was transformed into a battle ground, with sirens and bangs from flash bombs and firecrackers ringing out against the shouts of protesters. Eleven police suffered minor injuries,” as “breakaway groups of masked demonstrators fought running battles in the rain that left streets littered with makeshift barricades and debris.”

The Milan Expo 2015 is now open, and will remain so until October 31 2015.

Via Dezeen, Reuters