LONDON (AP) A goalkeeping blunder by David Ospina helped Olympiakos beat Arsenal 3-2 in the Champions League on Tuesday and leave the Gunners in real danger of its first group-stage exit in 16 years.

Ospina fumbled a corner over his line to give the visitors a 2-1 first-half lead, and substitute Alfred Finnbogason scored the winner in the 66th minute - almost immediately after Alexis Sanchez had drawn Arsenal level.

Arsenal, which has advanced past the first group stage every year since 2001, has now lost both its opening games - and still has two meetings with Group F leader Bayern Munich to come. Bayern beat Dinamo Zagreb 5-0 to take a tight grip on first place in the group.

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''It leaves us in a bad position, but we are still in it,'' Arsenal manager Arsene Wenger said. ''We have to make a result in our next game against Bayern at home. We are not out of it.''

Felipe Pardo gave Olympiakos the lead in the 33rd, but Theo Walcott equalized two minutes later as Arsenal looked likely to start an immediate fightback. However, the London club couldn't prevent Olympiakos from securing its first ever European win in England in 13 games - which included three group-stage losses at Arsenal in the last six years.

''The key factor today was our endurance,'' Olympiakos coach Marco Silva said. ''We ran a lot, we knew how to battle and how to remain determined to hold onto the result we deserved until the end.''

Wenger decided to rest first-choice goalkeeper Petr Cech, just as in the 2-1 loss at Dinamo in the first round, and it proved a costly decision.

Ospina misjudged an inswinging corner in the 40th minute and had to backtrack before failing to hold on to the ball, letting it bounce on the line. The extra official behind the extended goal-line ruled the ball had crossed completely, although some TV replays seemed inconclusive. There is no goal-line technology in use in the Champions League.

Wenger bristled after the game when he was asked about the decision to rest Cech.

''I don't have to sit here and give you an explanation of every decision I make,'' the Frenchman said. ''Ospina played 19 games last year and kept 14 clean sheets. ... No keeper is mistake free. It could have happened to Petr Cech as well.''

Olympiakos' first goal also came from a corner, when Pardo was left completely unmarked outside the area, and his shot took a deflection to wrongfoot Ospina.

Arsenal's reaction was swift, though, as Walcott took a pass from Sanchez and broke into the left side of the area, firing a low shot toward the far post that Olympiakos goalkeeper Roberto got his hands to but could only deflect into his own net.

Still it was the Greek side that went into halftime in the lead after Ospina's howler, and the two teams traded quick goals again in the second half.

Walcott turned provider for Sanchez this time as he lofted a cross into the area and the Chile striker leaned forward to head in his fourth goal in two games. But Olympiakos was back ahead a minute later after some more poor defending from Arsenal.

Pardo set it up this time as his cross from the right flank found Finnbogason, who beat Per Mertesacker to the ball and slotted his effort past Ospina.

Mertesacker had come on as a substitute after Laurent Koscielny went off injured.

With Olivier Giroud suspended, Wenger only had the rarely used Joel Campbell to throw on in search of a late equalizer, but the Gunners didn't come any closer than a free kick from Mesut Ozil that was punched away by Roberto.

Walcott had a last chance in injury time when he broke into the area but shot well wide.