Around 300 demonstrators, using banners reading "fight the power" and "we resist!" as shields, threw flares and Molotov cocktails at baton-wielding police in front of La Scala, where musical director Daniel Barenboim was taking his last opening-night bow.
Two policemen were reported to have sustained injuries during the clashes with the protesters, who were demonstrating against austerity in the recession-hit country as well as demanding rights for social housing following a series of squat evictions.
The season premier "Fidelio" marks the end of Barenboim's nine years at the prestigious opera house and the glitzy evening at the 18th-century theatre was attended by fashionista, show girls and leading politicians.
Milan's prefect Francesco Paolo Tronca, among the guests, slammed the "unacceptable violent protests against police" and the "hijacking of cultural events which serve to give strength and value to our city."
There was also little sympathy from president of the Lombardy region where Milan is situated, Roberto Maroni, who said "what a way to ruin a party."
"We always present the worst image of ourselves to the world. La Scala's opening night is a unique spectacle and we manage to ruin even that," he said.
But the demonstrators received some support from former minister of economic development Corrado Passera, who said there was "such a level of need among some parts of the population that it is wrong to get angry with those who protest."
The riot darkened an already uneasy season following a summer which saw Rome's conductor Riccardo Muti quit the Teatro dell'Opera amid bureaucratic difficulties and threats of strikes.