Images of the act show a human-sized doll hanging by its neck, wearing Vini Jr.'s #20 Real Madrid jersey, underneath a sign that reads "MADRID ODIA AL REAL" -- translated, it means "Madrid Hates Real".

It's believed Athletico Madrid fans were behind the disgusting stunt ... as the teams are playing each other Thursday night, and they're big rivals.

The bridge is located near Real Madrid's training complex.

Of course, most fans were horrified by the act ... and quickly alerted authorities, who immediately went to remove the mannequin and banner.

Police are now trying to identify who's responsible, and they're treating the act as a potential hate crime.

La Liga issued a statement denouncing the hanging in effigy, saying ... "We strongly condemn acts of hate against [Vini Jr.] Intolerance and violence do not fit in football. As on previous occasions, #LaLiga will urge the investigation of the facts in search of the conviction of those responsible, requesting the most severe criminal sanctions."

While it's believed Atletico's fans were responsible, the club distanced themselves from the act, and stressed the importance of respect within the rivalry.

"Club Atletico de Madrid communicates that such acts are absolutely repugnant and inadmissible and shame society. Our condemnation of any act that attacks the dignity of persons or institutions is categorical and unreserved. The rivalry between the two clubs is the greatest, but so is respect. No individual, whatever their intentions or colors, can tarnish the coexistence between different supporters."

"It is everyone's responsibility to avoid this. We do not know the perpetrator or perpetrators of this despicable act, but their anonymity does not avoid their responsibility. We hope that the authorities succeed in clarifying what happened and that justice helps to banish this type of behavior."

Real Madrid also released a statement ... saying the club "would like to thank the support and expressions of affection received after the unfortunate and disgusting act of racism, xenophobia and hatred against our player Vinicius."

"We express our firmest condemnation of events that violate the fundamental rights and dignity of people, and that have nothing to do with the values ​​that football and sport represent."

The club continued ... "These attacks like the ones our player now suffers, or those that any athlete can suffer, cannot have a place in a society like ours."