Celtic’s hopes of reaching the group stages of the UEFA Champions League suffered a huge blow after two freak goals gave Arsenal a 2-0 win in the first leg of the play-off.

A bizarre William Gallas goal handed the Gunners the lead on the stroke of half-time and Celtic’s task became even harder when Gary Caldwell deflected a cross into his own net to give the English side a priceless second away goal.

Arsenal, semi-finalists in last year’s tournament, showed all their class with some wonderful football during the 90 minutes, but the nature of the goals will have been hard to take for the Celtic players, who gave their all on the night.

Tony Mowbray’s side will now have to beat Arsenal by at least two clear goals at the Emirates Stadium next Wednesday to progress any further in the competition, although European football is guaranteed in the shape of the Europa Cup, regardless of the result in London.

After his match-winning heroics against Dinamo Moscow, Mowbray opted to start with Georgios Samaras in attack, with Marc-Antoine Fortune and Scott McDonald on the bench. Aiden McGeady was asked to support the Greek.

Scott Brown, now fully fit after a summer operation, was also in the Celtic starting line-up. The midfielder was deployed on the right side of midfield, with Shaun Maloney hogging the left flank.

Just a few miles down the road from Celtic Park, U2 were playing a sold-out gig at Hampden. Yet, there was only one show in town and Paradise was rocking by the time kick-off arrived.

The game was only seconds old when Brown steamed into his first tackle as Celtic started in lively fashion. Danny Fox sent a dangerous cross into the box early on, but there were no takers. Caldwell also headed over from a corner.

Celtic were competing all over the pitch, but Arsenal – impressively slick in possession – posted notice of their danger in 12 minutes when a sublime passing move ended with Andrei Arshavin drilling home. The Russian was correctly flagged offside – to the relief of the Celtic supporters.

That sparked a good spell for Arsenal and Massimo Donati was penalised for a foul on Nicklas Bendtner on the corner of the box in 19 minutes. Robin van Persie’s re-taken free-kick was cleared by Celtic and a moment of controversy followed.

Maloney picked up possession on the edge of his own box, jinked into space and sent McGeady racing away on the left as Celtic broke at speed.

He jumped one challenge and crossed for Brown, who was caught heavily by Thomas Vermaelen as he volleyed wide. The Celtic supporters screamed for a spot-kick but referee Massimo Busacca was having none of it.

The incredible pace of the game never dropped for a second during the first half and Artur Boruc was called into action for the first time in 40 minutes – the Pole making a low save from Arshavin after a weaving run.

Minutes later, Arsenal No.1 Manuel Almunia was tested when Brown pounced on a loose ball into the box and fired a low shot towards the corner. The Spaniard’s handling was excellent.

It had been an even game but Arsenal got a lucky break just two minutes before the interval. A free-kick was rolled short to Cesc Fabregas and his shot deflected off Gallas’s back and beyond the helpless Boruc.

Celtic were left reeling by that unfortunate concession and Arsenal attempted to take advantage at the start of the second half. Arshavin dispossessed Landry NGuemo and teed up van Persie, who rolled a shot wide of target.

Arsenal were pressing Celtic high up the field and Arshavin again picked up the ball before sliding a pass to Denilson. The Brazilian cut inside and curled a shot past Boruc’s left-hand post.

The Hoops had brief respite when a Maloney free-kick caused some panic in the Arsenal box and a penalty claim was again waved away. Yet, the Gunners kept coming and van Persie again came close with a low shot in 53 minutes.

Mowbray was looking for some fresh inspiration and sent on McDonald and Fortune for Donati and Samaras in 56 minutes. Within seconds, NGuemo was booked for a challenge on Fabregas.

Gallas, sliding to block a Fortune cross, sent the ball perilously close to his own net as Celtic searched for a way back into the game but the Hoops were rocked by yet another freak goal in 71 minutes.

This time, Arsenal broke up the left wing and a passing move ended with Gael Clichy firing a low cross into the box. Caldwell, trying to clear, could only divert the ball past Boruc.

McGeady, outstanding for Celtic, tried his best to spark a fightback but Arsenal gave nothing away in the closing stages. Mowbray’s men will now have to pull off a incredible result in London to keep their UEFA Champions League dream alive.