Two supporters were stabbed and an ambulance attacked by masked hooligans wielding weapons in Rome on Monday during a tense build-up to the Serie A derby between Roma and Lazio, reports said.
La Gazzetta dello Sport daily said some supporters clashed with police in the Duca d'Aosta area and two Roma fans were stabbed after an ambush on the Ponte Milvio, a bridge over the River Tiber which leads to the Olympic Stadium.
The report said it was "chaos outside the Olympic Stadium", where nearby shops closed early for fear of further clashes.
Rome mayor Gianni Alemanno pleaded with fans to calm down.
"We are very concerned," Alemanno told 24 Tg.com. "I'm appealing to fans to make this derby trouble-free."
ANSA news agency said medics driving an ambulance in the area were forced to flee after their vehicle was set upon by fans wielding a number of weapons.
The report said the Roma fans who had been stabbed also fled after initially receiving medical treatment.
It was unclear from the reports whether the ambulance was attacked for treating the stabbed fans.