They have, famously, one big clock at this stadium. Arsenal, rather less famously, are beginning to show signs of having two faces. Playing with the sort of resurgent, attacking verve that blew away Lazio so comprehensively here in September, Arsenal careered into a two-goal lead last night, their off-colour strikers Thierry Henry and Nwankwo Kanu at last finding their feet and form. There were 55 minutes gone and Arsenal's head was held high.

And then something happened, something that is not always obvious, something internal. Arsenal began to defend like men with nerves, rather than men with confidence.

In the 56th minute, Stefan Effenberg threw in a free-kick from the right. A posse of Arsenal defenders went for it. None met it convincingly. But Michael Tarnat did. Bayern Munich's left half drove the ball into Alex Manninger's bottom corner.

But Arsenal were still ahead and should have still been in control of their destiny. But no. Now their passing was scrappy when before it was accurate. Now they got tetchy. Freddie Ljungberg, for example, got himself booked. Previously he had been the added penetrative dimension that had helped Arsenal into the lead. "It gave the mental strength to Bayern," Wenger was to say.

Then, 10 minutes after Tarnat's lifeline, Ljungberg gave away another freekick. Because of Arsenal encroachment Bayern got to take it twice. The second time, Mehmet Scholl bent it into Manninger's top corner with the young goalkeeper rooted to the spot in near fascination as the ball glided by. In 10 swift minutes Bayern had undone Arsenal's labour, and Arsene Wenger's side never hinted that they were capable of re-establishing their superiority. They parted as equals.

But not in terms of the group. Bayern are top; Arsenal are bottom and Wenger must be relieved that the next match - Lyon away - is not until mid-February.

"It's very disappointing and very frustrating," Wenger said. "But you can't lose your concentration. I'm especially angry about the first goal. You can't defend like that at this level." He added: "I think we can still go through," but it was not said with the assurance we have come to expect from the Frenchman.

That just about sums up the Arsenal dilemma. Even with Dennis Bergkamp dropped to the bench, they started with the firepower of a controlled explosion. Yet they were to finish, discipline eroded, trying to pick fights with their opponents as they walked off.

How different it had been earlier. Only three minutes of cagey football had elapsed when Robert Pires collected the ball on the Arsenal left. Pires had little option but to cut inside, but immediately he did so Bayern were on the back foot. Pires fed Henry, running at his marker.

Instead of accepting the pass, however, Henry dummied the ball and motored on. Kanu saw him and slid the ball first time back into the Frenchman's path. For a man with only a penalty to his name in the past six weeks, Henry's finish was supremely confident. Oliver Kahn was a bystander. As a move it was almost the equal of Ljungberg's brilliant first against Lazio.

Arsenal would not have been flattered by a quick second, but were to be denied the most welcoming opportunity to get it, from the penalty spot in the 18th minute.

Referee Stefano Braschi waved play on when a typically cute piece of footwork by Kanu in the Bayern area fooled Ciriaco Sforza and saw him react instinctively with his arm.

If that was not bad enough, Bayern promptly broke away and only a sneaky foul by Patrick Vieira on Scholl halted their progress. The free-kick came to nothing. Well, that one did anyway. Elber had begun the attack with a slick flick, but that was virtually all that was seen of the Brazilian before the interval.

Ljungberg's profile gradually grew and it was he who won the corner in the 37th minute that Tony Adams met at the far post. Adams' header was goal-bound until Thomas Linke got his forehead in the way and deflected the ball onto the upright.

A useful snapshot by Scholl, the halftime introduction of Paulo Sergio and an Effenberg surge suggested Bayern were about to charge, but if that was their intention then it was interrupted by a Vieira header, another Ljungberg probe and finally by Kanu's magnificent goal.

The Germans complained about its origin - a bouncing ball free-kick taken by Vieira - but when Henry cut back from the byline and passed to Kanu, there was still much for the Nigerian to do. Kanu did it, finding the smallest corner away from Kahn's fingertips. It was some finish.

That should have been the end of Bayern. But it was the opposite, and a night that began with a security alert that delayed the kick-off by 25 minutes finished with alarm bells surrounding Arsenal's progress in this competition.

Arsenal: Manninger, Vieira, Keown, Adams, Pires (Wiltord 76), Ljungberg, Henry, Grimandi, Luzhny (Lauren 81), Kanu, Cole. Subs Not Used: Lukic, Dixon, Bergkamp, Upson, Vivas. Booked: Ljungberg, Grimandi. Goals: Henry 4, Kanu 55.

Bayern Munich: Kahn, Sagnol (Sergio 46), Kuffour, Scholl (Zickler 83), Elber, Sforza, Effenberg, Jeremies, Tarnat, Salihamidzic, Linke. Subs Not Used: Dreher, Andersson, Weisinger, Fink, Santa Cruz. Booked: Sforza, Jeremies. Goals: Tarnat 56, Scholl 66.

Referee: S Braschi (Prato).