Sporting Lisbon have confirmed that a number of their players and staff were assaulted by a group of fans who forced their way into the club's training ground on Tuesday.
Reports in Portugal claim that around 50 individuals, who had covered their faces to avoid being identified, interrupted training and assaulted members of the squad.
"Sporting vehemently condemned the events registered today at the Sporting Academy," the club said in a statement.
"We can not in any way agree with acts of vandalism and aggression to athletes, coaches and staff, nor with attitudes that configure the practice of crime that in no way honour and ennoble Sporting.
"Sporting is not this, Sporting cannot be this. We will make every effort to establish full responsibility for what has happened and we will not fail to demand the punishment of those who acted in this absolutely pitiful way."
Sporting, who are preparing for Sunday's Taca de Portugal final with Aves, failed to qualify for next season's Champions League after a defeat to Maritimo cost the club a second-place finish in the Primeira Liga.