Spanish police on Thursday said they had arrested four males suspected of inciting an online hate campaign targeting Real Madrid star Vinicius before last month's derby at Atletico Madrid.

The viral social media campaign before the September 29 clash encouraged Atletico fans to arrive at the club's Metropolitano stadium and utter "degrading and discriminatory insults with racist connotations" against a Real player, police said.

Participants were urged to come wearing masks to thwart attempts to identify them, police added in a statement without naming Vinicius.

The investigation began at the start of this month following three complaints by La Liga and four males suspected of inciting hatred were arrested on October 14 and 15.

Further arrests cannot be ruled out as the investigation remains open, police added.

Opposition fans have singled out Vinicius for racist abuse at several La Liga stadiums since the Brazil forward, 24, joined Real Madrid in 2018.

In January 2023, a dark-skinned effigy wearing a Vinicius shirt was found hanging from a bridge near the club's training ground alongside a banner saying: "Madrid hates Real".

In December, four Atletico Madrid fans were charged over the incident, with prosecutors pushing for jail terms of four years.

Last month's Madrid derby was also marred by a 15-minute suspension after Atletico fans hurled objects including lighters onto the pitch in the second half.

The violence eventually earned Atletico a fine and a partial stadium closure. The game ended 1-1.