Firefighters try to extinguish a fire during a rally against Expo 2015 in Milan, Italy, May 1, 2015. Italy opens the Milan Expo on Friday, torn between hopes that the showcase of global culture and technology will cheer up a gloomy national mood and fears that it will be overshadowed by scandal, delays and street protests. [Photo/Agencies]

Milan's anti-terrorism prosecutors opened an investigation into allegations of major acts of vandalism, which carry a maximum prison sentence of 15 years, against five people who have been arrested and other as-yet-unknown perpetrators.

With local and regional elections bringing about 17 million Italians to the ballot box later his month, opposition Northern League and the anti-establishment 5-Star Movement both pressed Interior Minister Angelino Alfano to resign.

Opposition leaders said Alfano, the leader of a centrist bloc allied with Renzi's Democratic Party, failed to prevent the violence and damage, despite widespread expectations of trouble.