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The alleged incident occurred moments after Vinicius had curled Madrid ahead five minutes into the second half at the Estadio da Luz, before referee Francois Letexier halted the match for 11 minutes under FIFA's anti-racism protocol.

Television pictures showed the Argentinian winger covering his mouth with his shirt before making a comment that Vinicius and nearby teammates interpreted as a racial slur against the 25 year-old, who has repeatedly suffered racism in Spain during matches.

"It is unbelievable that, with dozens of cameras in the stadium that none could have caught (Prestianni’s) racial slur, but if you cover your mouth to say something, that says a lot..." Madrid captain Federico Valverde said.

"According to my teammates who were close, they heard something very ugly. Many people have been fighting against racism in football, Vinicius is one of them. I’m proud of him and his great game."

Benfica began on the front foot but the visitor gradually assumed control while the hosts appeared content to counter-attack, with goalkeeper Thibaut Courtois producing a stunning one-handed save to keep out a 25th-minute strike from Fredrik Aursnes.

Madrid finished the first half strongly and only goalkeeper Anatoliy Trubin's brilliance kept the score goalless at the break, as he produced stunning saves to deny Kylian Mbappe and Arda Guler.

The breakthrough came five minutes after the restart. Mbappe burst forward and fed Vinicius on the left. The Brazilian cut diagonally towards the box, shifted inside and unleashed an unstoppable curling shot into the far top corner.

Vinicius celebrated by dancing with the corner flag and was booked by Letexier for excessive celebration.

As the forward argued with the referee, he became involved in a heated exchange with Prestianni, with Vinicius and several teammates suddenly running towards the referee.

Letexier crossed his arms in front of his face to activate FIFA's protocol and stopped the match. Real's players threatened to leave the pitch as tensions flared before play eventually resumed 11 minutes later.

The atmosphere turned hostile, with Vinicius loudly booed whenever he touched the ball and the broadcast camera appeared to show an outraged Mbappe calling Prestianni "a bloody racist" to his face several times after the game resumed.

Tempers flared again in the 85th minute when Vinicius fouled Richard Rios. Benfica manager Jose Mourinho stormed towards the fourth official demanding a second booking for the Brazilian but was himself cautioned for his protests.

An incensed Mourinho continued remonstrating and was promptly shown a second yellow card.

Benfica pushed late on but managed only three shots on target as Real secured a slender advantage to take back to Madrid for the return leg.