Benfica's title celebrations in Lisbon on Sunday evening were marred by clashes between supporters and police.
A goalless draw at Vitoria Guimaraes earlier in the day had earned Benfica consecutive Primeira Liga titles for the first time in 31 years.
The match was held up for two minutes as flares were thrown on to the pitch.
And the Benfica players, arriving back in central Lisbon in the early hours of Monday morning, were greeted by jubilant crowds in Marques de Pombal Square, although the celebrations turned sour as violence broke out.
Portuguese newspaper Abola said police used force after a number of fans began throwing objects.