Brazilian soccer star Dani Alves was reportedly detained Friday in Spain ... where he's being held on sexual abuse charges.
Alves, 39, is accused of inappropriately touching a woman under her clothes without consent (in Spain, sexual assault can cover anything from groping to rape) in a Barcelona nightclub on New Year's Eve, according to reports.
The woman filed a report after the alleged incident ... and once local police were made aware of the situation, they took witness statements.
Alves -- a champion with several clubs, including Barcelona, Juventus and Paris-Saint German -- has denied any wrongdoing, claiming he's never even met the alleged victim.