Protesters with covered faces and helmets blocked roads and set fire to garbage bins near one the capital's universities on Tuesday.
Police spokesman Vittorugo Caggiano said 11 protesters were detained under various charges, including a French, a Polish and a German citizen. Caggiano said the rest would be released later Tuesday.
Separately, police in L'Aquila seized a baseball bat and metal bars from the car of five French protesters. They were not arrested.
Several protests have been planned in Rome and in L'Aquila, which will host the Wednesday-to-Friday summit.