LONDON: Arsenal's hopes of advancing from the Champions League group stage for the 16th straight season were left dangling by a thread after they lost an astonishing clash 3-2 to visiting Olympiakos Piraeus on Tuesday.

The Gunners have now lost their opening two group matches for the first time ever following their 2-1 defeat at Dinamo Zagreb two weeks ago and are bottom of Group F, six points behind leaders Bayern Munich, who they have to play twice.

Olympiakos, whose 12 previous visits to England since 1965 all ended in defeat, scored as many goals in this match as they had managed in the last 50 years in England, and have three points along with Dinamo, who were beaten 5-0 at Bayern.

The defeat left Arsenal manager Arsene Wenger in a tetchy mood when he faced the media at the post-match news conference.

His team selection was criticised when they played in Zagreb and he came under renewed questioning after this loss especially about his decision to play No.2 stopper David Ospina instead of first choice Petr Cech. He was unusually curt with his answers.

"I make the decisions and the selections for the team and I am responsible for it. I know many things that maybe you don't know and maybe ignore," he said.

"You cannot select the team by making a poll before the game and getting everyone's opinion."

"It leaves us in a bad position but we are still in it," added Wenger. "We have to think we can beat Zagreb here and Olympiakos away from home and get a result in our next game against Bayern at home, but we are not out of it."

GREEK LEAD

Olympiakos, who had lost their last five Champions League away games and their last six in all European competition took the lead against the run of play when Felipe Pardo's 32nd minute shot was deflected by Alex Oxlade-Chamberlain past Ospina.

Arsenal were level three minutes later when Theo Walcott scored from a tight angle but Olympiakos went in front again when Ospina mishandled a corner and dropped the ball over his line with the fifth official signalling a goal.

Alexis Sanchez equalised for the hosts with a neat header in the 65th but almost immediately Icelandic substitute Alfred Finnbogason put Olympiakos ahead again.

The visitors carved their way through an unusually shambolic Arsenal defence, disrupted by the loss of Laurent Koscielny who went off with a hamstring injury after 57 minutes.

A key moment came though when Ospina made a calamitous mistake to gift Olympiakos a 2-1 lead five minutes before the break, the keeper failing to collect an inswinging corner when unchallenged and allowing the ball to drop over the line.

"It's always a simple thing to say Ospina should not have played. He played 19 games last season and kept 14 clean sheets and last week at Tottenham he had a fantastic game," said Wenger.

"No goalkeeper is mistake-free -- it could have happened to Petr Cech as well. But I don't have to sit here and give you an explanation of every decision I make. We did not lose the game because of that mistake, that's a farce."

Ospina's own goal was the third goal in an eight-minute spell after Pardo and Walcott traded goals.

The next arrived when Sanchez headed in Walcott's precise cross in the 65th but a minute later Finnbogason finished off a neat passing move to with what proved to be the winner and left Arsenal with a mountain to climb to reach the last 16 again.