Protesters are seen past a burning 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]

MILAN - Italian prosecutors opened an investigation on Saturday into rioting in Milan that disrupted the start of Expo 2015, while opposition parties called on the interior minister to resign.

Twenty-four hours after the riot, in which cars were torched, street fittings wrecked and buildings damaged by fire and defaced by spray paint, much of the visible damage had already been cleaned up by citizens and municipal workers.

The violence marred the opening day of the Expo, a global fair focused on the theme of sustainable food production, that Prime Minister Matteo Renzi said would help put a new face on Italy after years of economic decline.