Renato Gaucho believes Gremio were "robbed" by VAR in their controversial Copa Libertadores semi-final elimination at the hands of River Plate on Tuesday.

Defending champions Gremio secured a 1-0 advantage in the first leg at El Monumental but fell to a 2-1 defeat in incredible circumstances in the return match in Porto Alegre to go out on away goals.

Leo Gomes extended Gremio's aggregate lead in the first half before Rafael Santos Borre made it 1-1 on the day in the 82nd minute, though Renato felt the goal should have been disallowed for handball by the Colombian.

The Brazilian side were reduced to 10 men in the 88th minute when referee Andres Cunha deemed, following a review of video footage, that Bressan deserved a second yellow card for blocking Ignacio Scocco's shot with his arm in the box.

The furious centre-back had to be restrained as he grasped hold of the referee in protest, and, almost 10 minutes after the initial incident, Gonzalo Martinez fired home the penalty to send River into the final.

"Gremio only haven't qualified because of VAR," said an irate Renato after the match.

"If it worked, I would be smiling, the fans would be happy, and Gremio would be in the final of the Libertadores. Everything would be right.

"Gremio were robbed. I don't think there's any doubt about the [Borre] incident.

"It's the referees who teach us. How come they don't know how to use it? It's one thing not to understand, another thing is to be blind and another thing is to be dumb.

"I don't think he's stupid. He's there to see the play, he gets paid for it, there are many cameras available for him, there are a lot of people who can't even imagine how many cameras he has available. There's no way he can be wrong.

"In a play like that, he should have called the referee to review the play. And surely he would see that the goal was with the player's arm. He would have cancelled the goal."

River boss Marcelo Gallardo watched from the stands due to a suspension but could face punishment from CONMEBOL following the emergence of video footage allegedly showing him leaving the team's dressing room after half-time.

Gallardo was proud of his side for overturning the first-leg deficit and securing a meeting with either Boca Juniors or Palmeiras in the decider, their arch-rivals holding a 2-0 aggregate lead in that semi-final.

"We won a historic match," said Gallardo. "We went out to attack because we were at a disadvantage and needed to win the game. The first half was very good.

"One team was serious and the other one tried to defend. I am happier with the victory because it shows Gremio had to respect us throughout the tie."