Barcelona have criticised the "disproportionate use of force" used against the club's supporters by the Spanish police before Sunday's LaLiga opener against Elche.
A video shared on social media showed law enforcement agents aggressively hitting fans with batons outside the Estadio Manuel Martínez Valero after confiscating Estelada flags.
The Estelada is a pro-independence flag associated with Catalonia, one of the 17 autonomous communities in Spain.
"Barça condemn the disproportionate use of force against supporters during the incidents that took place in the vicinity of the stadium ahead of the league game against Elche," the club said in a statement on Monday.
"In particular, the club considers that the procedure used to detain one of our fans was disproportionate and that the use of force lacked the necessary justification and appropriateness.
"Barça also regret that, once again, police officers confiscated Estelada flags from fans.
"The Estelada is a legally endorsed symbol and its display constitutes a legitimate exercise of freedom of expression. As such, it cannot in itself be considered an incitement to hatred or violence."
The incidents have also been condemned politically by Junts, the main pro-independence opposition party in Catalonia.
"They have a deep, systemic hatred of Catalan identity," Junts leader Carles Puigdemont posted on social media alongside images of the police beating Barça fans.
"This affliction extends from people in the highest echelons to football players, as well as judges and journalists. In a healthy society, this would be intolerable. In Spain, it is considered a virtue."
"These animals have to pay," the leader of Junts in the Spanish Congress, Míriam Nogueras, added.
Both Barça and Junts have offered support to the detained fan, with the Catalan club also threatening to take the issue further if required.
"Barça await the relevant explanations regarding these events and reserve the right to take whatever action it deems appropriate to defend the integrity, rights and freedom of expression of its fans, both at Spotify Camp Nou and at opposing teams' stadiums," Monday's statement said.
Barça won the game in Elche 5-0, with Raphinha and Fermín López both scoring twice and Karim Adeyemi also netting on his debut as the Spanish champions kicked off their defence of the title in style.
The victory was tainted by a worrying knee injury sustained by Gavi in the first half, but tests back in Barcelona on Monday revealed no major issues.
"Medical tests carried out this morning have confirmed that it is only bruising," Barça said. "So there is no cause for alarm and he is expected to return to training shortly."