Fabianski gives Porto advantage
The Polish goalkeeper, deputising for the injured Manuel Almunia, was caught out of position to gift Porto an early lead, before then picking up a back-pass which led to another goal at the start of the second half.
However, Sol Campbell's header which had levelled at 1-1 could yet prove to be crucial when the teams return to the Emirates in a fortnight's time.
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Hide AdArsenal manager Arsene Wenger accused referee Martin Hansson of a "massive mistake".
Wenger was furious that Hansson awarded Porto the indirect free-kick. 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.
"I believe it (the back-pass) was accidental, Sol touched it accidentally," he said. "And the referee should give us time to build the wall." Asked if Porto should have been allowed to act so quickly, Wenger added: "Yes, in the middle of the field, but not in the box. We had no chance to defend."
Hansson was also the referee who missed Thierry Henry's handball for France in their World Cup play-off.