Forty-eight people were injured as fans clashed outside the Dragão stadium before Sunday’s Portuguese Premier League game between Porto and Sporting Lisbon, police said on Monday.
About one hundred fans were involved in the pre-match disturbances and were taken to a police station, but then released.
“In a very cohesive and organised way, the group provoked the violence and forced their entry near the stadium’s gate 25,” the police said in a statement.
TV footage showed the problems started when a group of people, mostly dressed in black and not wearing any club gear, arrived in formation at the premises outside the Dragão stadium chanting and provoking Porto fans standing around.
Fighting broke out and the situation only calmed down after police intervened using batons.
Some people had to be treated in hospital but there were no reports of serious injuries.
“The individuals who caused the violence were intercepted inside the stadium, after which they were taken to the police department in order to be identified and inquired about the circumstances in which they committed those actions,” police said.
Porto beat Sporting 3-1 and extended their lead at the top of the championship.