Arsenal coach Arsene Wenger said referee Martin Hansson made a 'massive mistake' in allowing Porto's controversial second goal in the 2-1 win over the London side in the Champions League last 16 first leg on Wednesday.

Wenger was left fuming at the referee for the 51st-minute goal on after his side were caught napping when Ruben Micael quickly took an indirect free kick in the box to give Radamel Falcao the easiest of tap ins. "We had no chance to defend. I think it was a massive mistake, a double mistake," Wenger told reporters.

The Swedish referee, who was at the centre of the storm around Thierry Henry's handball in France's World Cup playoff against Ireland last year, penalised a Sol Campbell back pass to keeper Lukasz Fabianski and then allowed the free kick to be taken instantly.

"I believe it was accidental. It was not a back-pass, Sol touched the ball accidentally. The referee then pushes the goalkeeper away, and gives him (Ruben Micael) the ball and they score a goal," Wenger added.

The Frenchman said the referee should have given Arsenal time to build their wall. He had a heated exchange with the referee at the time but said he did not know why the goal was allowed.

Wenger added Porto would have been allowed to play on, in the letter of the law, if an indirect free kick was awarded in the middle of the pitch -- not in the penalty area.

Porto coach Jesualdo Ferreira had words of sympathy for Wenger but said the goal was legitimate.

"This happens to all coaches, and it has happened to me before. And we don't know who to be angry at, the referee or our players. If the goal wasn't legal then we could discuss the thing another way, but it was and the law permits it," he said.

"It was an intelligent move by my player, an easy but opportune finish by Falcao, and it gave us a victory that was fair," he added.

Ferreira rued the goal Porto conceded at home.

"It's a result that favours Arsenal since they scored away, but I am convinced we will score goals in London."