Arsenal manager Arsene Wenger accused referee Martin Hansson of a ''massive mistake'' in his side's 2-1 defeat to Porto.
Wenger was furious that Hansson awarded Porto an indirect free-kick after Lukasz Fabianski picked up Sol Campbell's backpass. And the official then appeared to get in Campbell's way as Porto were allowed to take a quick free-kick to score their second goal.
"It is difficult to swallow a defeat like that,'' said the Arsenal manager. "The goals we conceded were difficult to imagine in a Champions League game.
"What can you do about the second goal? The back-pass was accidental, whenever do you see the defender kick the ball back with his toe? The ball hit Sol, it was not on purpose and it has to be intentional to be a free-kick. It is difficult to understand how the referee can interpret that.
"Then, on an indirect free-kick, if you allow the team to play quickly, just five metres from the goal, how can you defend that? It is better than a penalty. It was unbelievable that he allowed Porto to play straightaway and push the ball into the net.
"I have never seen that and I have been in the game a long time. It is difficult to understand. It is completely inappropriate that he allows that in such a situation. When the referee gives the free-kick he has to allow us a chance to defend it, otherwise it is better to give a goal straight away.
"From a situation where there was no goal chance at all, and no free-kick as well, the referee gives them just a goal. It is difficult to understand, but maybe I am not intelligent enough.
"I do not want to come out individually on Lukasz's performance, and judge him in front of everybody. You have to accept you lose as a team and win as a team. Any individual performance has not to be analysed publicly.''
Despite the defeat, Wenger maintains all is not lost in the tie.
"It is unfortunate we lost this game, but we are still in the tie,'' said the Arsenal boss. "We have a good opportunity to turn it around in the second game.
"There was a strong penalty claim for us on Rosicky which was turned down, and the repeated fouls in midfield were not punished enough. But Porto played well, and we have a chance to turn things around in the second game. That is what we will try to do, and I am convinced we will do it.
"Sol scored an important goal which can prove to be vital now in qualification. We will give everything in the second game at home.''
Arsenal captain Cesc Fabregas conceded he might well have taken a quick free-kick under the same circumstances.
''Maybe as a player I would have done the same to try to score,'' he said. And the Spaniard admitted the Gunners also had themselves to blame.
''When you concede these goals you cannot go anywhere, schoolboy goals, what can you do?'' he added. ''Maybe we are still a little soft in that aspect, as a team when we concede we are not strong enough to lift ourselves, we were not strong enough to stand up and play well.''
Porto coach Jesualdo Ferreira, though, had few complaints about the manner of his team's victory.
He said: "It was a legal goal, one born out of the intelligence of a Porto player. Thierry Henry also did the same thing when he played for Arsenal.
"It gives a definite advantage to Porto now in the tie.''