Protesters flip over a car 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]

Most of the protesters marched peacefully, criticising the Expo as a symbol of waste and corruption.

But when a minority of masked, black-clad protesters began to throw petrol bombs and set fire to cars and bank branches, police sought to isolate them instead of rushing in to stop the destruction with force.

"Four little hooligans with silver spoons in their mouths will not succeed in ruining the Expo," Renzi said in an interview with RAI state TV. Police "did their job seriously and avoided provoking" the demonstrators, Renzi said.

The 5-Star Movement said it would present a motion of no confidence in parliament against Alfano.