A controversial Radamel Falcao effort gave Porto a 2-1 win over Arsenal in their Champions League last-16 first leg but the English side will take heart from Sol Campbell's away goal on his return to the tournament after four years.
Arsenal coach Arsene Wenger was left fuming at referee Martin Hansson for the 51st-minute goal on Wednesday after his side were caught napping when Ruben Micael quickly took an indirect free kick in the box to give Falcao the easiest of tap ins.
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 Campbell back pass to keeper Lukasz Fabianski and then allowed the free kick to be taken instantly.
Porto had started strongly and their attacking intent paid off in the 11th minute when Fabianski bungled Silvestre Varela's cross into his own net.
But the Portuguese champions gave Arsenal too much room right after the opener and the English side took advantage to equalise.
Campbell, playing in the Champions League for the first time since scoring for Arsenal the 2006 final, was left unmarked at a corner and headed in seven minutes later.
Porto could have taken the lead again on 28 minutes when midfielder Micael's drive was blocked by Fabianski.
Arsenal's back four improved after the half-hour and, with Porto were struggling to defend in the air, a Nicklas Bendtner header forced Helton to tip over the bar.
Falcao's controversial goal shook the Arsenal side and they were unable to create clear chances until the end of the game, with Theo Walcott's introduction on 68 minutes adding little bite.
Porto searched briefly for a third goal, with Hulk and Raul Meireles forcing Fabianski to tight saves, but tired in the last 15 minutes and did not pose much of a threat either as the game petered out.