Around 50 supporters with covered faces interrupted practice at Sporting's training centre in Alcochete, 20 kilometres south of Lisbon, and assaulted players. Dutch striker Bas Dost sustained injuries to the head.
Portuguese football was rocked on Tuesday when a large group of fans forced their way into Sporting Lisbon's training centre and assaulted its players and staff.
Public television RTP reported that around 50 intruders, who had covered their faces to not be identified, interrupted practice at Sporting's training centre in Alcochete, about 20 kilometres south of Lisbon.
RTP showed images of bloody wounds to the head of Sporting's Dutch striker Bas Dost and of a changing room that had been trashed.
Police detained 21 suspects in the hours following the incident, according to Assistant Secretary of State for the Interior Isabel Oneto.
This type of "hooligan" violence that has plagued football in other European countries is unprecedented in Portugal.
Sporting issued a statement condemning what it calls "acts of vandalism and the attacks on (our) athletes, coaches and professional staff."
The club did not immediately respond to calls and emails.
The Portuguese league also condemned "the violent acts the players and coaching staff of Sporting Lisbon were subject to today."
"The perpetrators of these incidents are not fans of football, they are criminals," the league said in a statement.
Later Sporting fans took to the streets under the slogan "50 is not 3.5 million" in reference to non-violent fans of the club.
Sporting missed out on a Champions League berth for next season after a loss in the last round of the Portuguese league cost it a second-place finish last weekend.
It is preparing to play the Portuguese Cup final against Aves on Sunday.
Oneto said that authorities would increase security measures for the cup final.
The rampage marks the low point of an already tumultuous season for Sporting.
Club president Bruno de Carvalho launched a social media tirade against several of his own players, calling them out by name, following a defeat last month. The players responded by demanding respect and coach Jorge Jesus had to deny publicly that his players had been suspended by the club.