ZAGREB, Croatia (AP) Dinamo Zagreb swept to a surprising 2-1 victory over 10-man Arsenal in the Champions League on Wednesday as the Londoners' made a dismal start to their campaign in Group F.

A goal by Josip Pivaric in the 24th minute and a sending-off for Arsenal striker Olivier Giroud after a second yellow card in the 40th swung the game Dinamo's way before the break.

Dinamo's Chilean forward Junior Fernandes headed home in the 58th and although substitute Theo Walcott pulled a goal back for Arsenal in the 79th minute, it was too little too late for Arsene Wenger's side.

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Dinamo had never beaten a Premier League team before and had gone 15 matches without a victory in the competition, including qualifiers. However, it had not lost in a formidable run of 41 matches and last season became the first team to go unbeaten through a Prva Liga campaign.

''We were a bit unlucky tonight,'' Wenger said. ''I believe the first goal is offside and after the sending off is very harsh.

''We were not at our best, but nor was the referee tonight.''

The outcome will be a bitter disappointment for an Arsenal side which is realistically only expected to be the group runner-up behind Bayern Munich, which beat Olympiakos 3-0 in Piraeus.

''Mathematically, we can qualify without any problem if we produce our performances that we want at home,'' Wenger said.

''Overall, I believe we have problems to start in every competition this season,'' he added. ''We had problems in the Premier League and again in the Champions League tonight, but I am confident we can put that right in the next five games.''

Wednesday's defeat also came despite Arsenal making a bright start.

Giroud threatened with a powerful header in the ninth minute and then hit the post with a close-range effort after Oxlade-Chamberlain had broken through on the right flank and pulled an inviting ball back for the France striker.

Instead, Dinamo ended the deadlock with its first real attack, with Algerian forward El Arabi Hilal Soudani threading a superb ball through the Arsenal defense for Pivaric.

The Croatian midfielder's shot was parried by Arsenal goalkeeper David Ospina, but the ball rebounded off him as Oxlade-Chamberlain closed in to try and clear the danger.

Things went from bad to worse for the visitors when Giroud was shown a second yellow card for tripping Daniel Ivo Pinto.

Dinamo looked to press home its advantage and almost doubled its lead at the start of the second half when Soudani glanced a header against the far post, with the rebound scrambled behind.

But Fernandes made no such mistake with his header from a corner from the left, getting ahead of Arsenal defender Laurent Koscielny and Ospina to nod the ball home.

Walcott reduced the deficit when he produced a cool finish after sprinting clear onto Alexis Sanchez's pass down the right, but there was to be no late drama despite some five minutes of added time.

''I am very happy and proud,'' Dinamo Zagreb manager Zoran Mamic said. ''I thank the team for doing what I told them and the fans for pushing us when Arsenal had us under pressure.

''Now they have such confidence because see how good they really are.''