Gunners boss Arsene Wenger made half a dozen changes from the team which had suffered a shock 3-2 home defeat by West Brom last weekend, ending their unbeaten start to the domestic season.
However, after the players had to change in the dark following a power failure before weathering an early onslaught from Partizan, who were spurred on by a ferocious support, Arsenal snatched the lead through Andrey Arshavin, but then wasted several chances before Cleo levelled from the spot.
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The Gunners regrouped for the second half and eventually their pressure told when Partizan defender Marko Jovanovic was sent off for hauling down Marouane Chamakh.
Although Arshavin missed the resulting penalty, headed goals from Chamakh and Sebastien Squillaci, his first since arriving from Sevilla, gave the visitors a two-goal lead, which was preserved when stand-in keeper Lukasz Fabianski - deputising for the injured Manuel Almunia - saved a late penalty as Arsenal recorded a first European away win in more than a year.
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Captain Cesc Fabregas and centre-half Thomas Vermaelen could return forSunday's trip to the Barclays Premier League leaders where Sagna knowsArsenal's title credentials will again be under the spotlight.
'We had to win against Partizan because we did not play well at the weekend, conceding three goals,' the France full-back said.
'Every game is a test because the season is long, where you have to play match after match.
'Now we need to stay focused and look forward to the next game, and we have to make something at Chelsea. We have to play our football.
'We know we have the quality to beat them. It will be an open game, but it will be about us, not them.'
After one of the floodlights failed following a series of power cuts in the main stand, there was some doubt whether the match would actually go ahead before the referee got agreement from both sides.
Sagna revealed the Arsenal players saw the funny side of things, but got serious as soon as they went on to the pitch.
'The lights went off, so we had to use the lights from the phones to get ready, he said. It was quite funny, but we stayed focused and came on the pitch to get the win.'
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