Mobile phone footage has captured around 50 hooded individuals in the Portuguese town of Alcochete. The group allegedly broke into Sporting Lisbon's training complex on Tuesday before attacking players and coaching staff.
The intruders, believed to be Sporting supporters, were reportedly armed with sticks and belts, catching players by surprise in their changing rooms.
The club condemned the "acts of vandalism and aggression".
Players such as Rui Patricio, William Carvalho and Coach Jorge Jesus were among the targets.
João Palhinha, 22, was the only player to speak to journalists. He told reporters: "it's a very sad day for us."
Sporting Lisbon are coming to the end of a difficult season after finishing third and missing out on a Champions League qualification on the final day.
The incident has made headlines across the world's media outlets including the UK's 'The Guardian', 'The Marca' in Spain and 'The Kicker' in Germany. French media posted images of head injuries allegedly suffered by Dutch International Bas Dost during the incident.
Meanwhile, Sporting fans rallied round their team on social media, organising demonstrations of support in Alvarde.
Their side is currently preparing for this weekend's Portuguese Cup final against Aves and will be hoping to put this incident behind them.
The Government has confirmed 21 people have been arrested so far in connection with the incident.