Andres Iniesta's injury-time winner secured victory for Barcelona after going behind to Scottish champions Celtic, while Chelsea face a challenge to defend their title after defeat against Shakhtar Donetsk.
Champions League round-up: Celtic left smarting after Barcelona steal late victory
Neil Lennon admitted it was a "sore one to take" after a cruel injury-time goal from Jordi Alba gave Barcelona a dramatic late victory over Celtic in their pulsating Champions League Group G clash in the Nou Camp.
Georgios Samaras' 18th minute header from a Charlie Mulgrew free kick went in off the back of Javier Mascherano to give the visitors a shock lead, but a minute before the break Barcelona midfielder Andres Iniesta levelled.
With the four minutes of added time almost played at the end of an exhausting evening in which Parkhead keeper Fraser Forster had been outstanding, defender Jordi Alba broke Celtic hearts when he sneaked in at the back post to knock an Adriano cross past Forster from close-range.
"I thought the team were magnificent," the Celtic manager said afterwards.
"It is bittersweet really, it is hard to take losing the game in the manner in which we did but I am so proud of the players, I think they gave the club and country a huge shot in the arm.
"The players are hurting, they are angry, whatever you want to call it but it gave them a real a taste of playing in the Champions League at the highest level and they played fantastically well.
"No one gave us any hope but I think we have won a lot of admirers.
"I hope they take plenty from the game. It is tough to play at this level. But there are no recriminations and no criticism. I think we leave with our reputation intact."
Celtic remain second in the group with four points from three games ahead of the return game against Barcelona in a fortnight's time.
In the same group, Spartak Moscow earned their first Champions League points of the season with a 2-1 victory at home to Benfica.
Rafael Carioca gave the hosts a third-minute lead, and although Rodrigo Lima headed Benfica level at the Luzhniki Stadium, a Jardel own goal restored Spartak's advantage before half-time.
Roberto Di Matteo admitted Chelsea must avenge their Champions League defeat at Shakhtar Donetsk in two weeks time if they are to avoid one of the shortest defences of the trophy.
The European champions crashed to a 2-1 defeat to Shakhtar this evening as John Terry's return to the side went awry at the Donbass Arena to leave them facing a real fight to qualify for the knockout stage.
The match marked the first appearance of Terry since accepting his domestic racism ban, but got off to the worst possible start as he unwittingly set up Shakhtar's opener from Alex Teixeira inside three minutes.
Fernandinho then put Shakhtar even further ahead seven minutes into the second half, with Oscar scoring a late consolation for the visitors.
Previous holders have never failed to qualify for the knockout stage, and Di Matteo acknowledged Chelsea now a tough challenge to maintain that record.
"We need to win our two home games remaining for sure," he said.
"The consolation tonight is that Juventus drew in Denmark. It just shows how the Champions League is a very competitive competition.
"But certainly, we need to win our home game against Shakhtar."
Juventus are four points adrift of Shaktar in the group following a surprise 1-1 draw at Nordsjaelland.
Mikkel Beckmann's free kick gave the hosts a shock lead five minutes into the second half, and they held out for more than half an hour before Mirko Vucinic lashed in an equaliser.
Galatasaray claimed the first point of their campaign with a 1-1 draw against Cluj.
It was a frustrating night for the hosts, with Dany Nounkeu putting through his own net to give Cluj a 19th-minute lead before Felipe Melo missed a penalty 10 minutes before half-time.
Galatasary dominated the second half though, and got their reward through Burak Yilmaz 13 minutes from time.
Group F is a much tighter affair with Valencia, BATE Borisov and Bayern Munich all locked on six points.
A hat-trick from Roberto Soldado ensured Valencia were convincing 3-0 winners in Belarus.
The Spain striker opened the scoring from the penalty spot just before half-time and struck twice more in the second half to sink BATE.
Bayern, meanwhile, hit back from their shock loss to BATE last time out to edge past Lille 1-0, courtesy of Thomas Muller's 20th-minute penalty.
The result means Lille have lost all three of their matches so far, conceding six goals in the process.
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