Zlatan Ibrahimovic ended his ten-game goal-drought against English clubs at the Emirates Stadium last night.
But sub Theo Walcott led a dramatic fightback to put Arsenal right back in the tie after an hour of Barcelona magic.
Ibrahimovic, the Swede with a Bosnian mum and a Croatian dad, needed just 23 seconds after the break to give the Catalans the lead they deserved.
And 13 minutes later he added a second to all but end the need for a return leg at the Nou Camp next week.
But Walcott emerged with a goal to keep hope alive - and tormented Barca with his pace. With time running out, Arsenal produced yet another late goal - a Cesc Fabregas penalty - to equalise with five minutes left.
Walcott said: "It's been very frustrating for me not to start. I just wanted the chance to show what I could do. We've had a lot of disappointments this season, but the fans help us lift ourselves. If you look back on the game, they had a lot of the ball, I think 2-2 was a good result in the end."
Too right Theo. Arsene Wenger will be praying for a miracle of epic Red Sea-parting proportions over there.
He lost William Gallas 40 minutes into his long-awaited return to action, Andrei Arshavin was also injured and Fabregas misses out on his longed-for return to Barcelona after one of three first half yellow cards suffered at the hands of the over-fussy referee.
Arsenal's Premier League hopes will be hit too - Fabregas injured himself hitting the penalty too hard and limped through the last five minutes with no replacements left - he was only cleared to play minutes before the kick-off last night.
With both Barcelona centre-backs ruled out for the Nou Camp too, Wenger said afterwards: "My only regret is that we gave two soft goals away. I had to leave Cesc on, he wanted to stay on."
Quite how Arsenal survived a torrid first half without conceding nobody knows, except perhaps the magnificent Manuel Almunia. With a fussy referee blowing every touch, Arsenal simply never got into their stride. That they escaped with a draw is testament to their new-found ability to dig in.
Instead of seeing the Gunners pushing the ball around in neat triangles against hapless Hulls and Sunderlands, it was the home side who were left chasing shadows by the Spanish and European champions.
In the first 20 minutes, with Messi and Ibrahimovic running riot, the much-derided Manuel Almunia made five superb saves. After 17 minutes, Arsenal were finally allowed their first attack.
Never have I seen Arsenal outplayed like that at home, not in 35 sometimes very mediocre years. By the time Samir Nasri had Arsenal's first shot in the 22nd minute, the Catalans had enjoyed 12 attempts, six on target.
Almunia, blamed for Kevin Phillips' late equaliser for Birmingham on Saturday, received support from watching former Arsenal No1 Jens Lehmann. He said: "Almunia made saves like I haven't seen any goalkeeper make this season."
But having scrambled to half-time with the problems mounting, Ibrahimovic needed less than half a minute to put Barca ahead after the interval, beating the off-side trap to find the far corner of Almunia's net.
It was inevitable. By then, Arshavin had been booked and limped off injured for Emmanuel Eboue. Alex Song, Arsenal's best midfielder, was also booked - and then moved to centre-back when Gallas was carried off with a recurrence of his calf problem.
And on came Denilson, the inneffectual Brazilian, to replace Song, the rock in midfield.
The writing was on the wall. It just needed Ibrahimovich to read it.
And he did. Brilliantly.
The first was a break and finish from a narrow angle. The second, in the 58th minute, saw him narrowly escaping the off-side trap again to lash the ball into the roof of the net and emphatically end this tie as a contest long before the return to Spain.
Between those two hammer blows, Nicklas Bendtner, way out of his depth in this company, managed a solid header which briefly troubled Valdez. But for Arsenal that was as good as it got until the arrival of sub Walcott, Arsenal's only English player on the night, down the right.
I sent an ironic message to a fellow Gooner James Goldman on Facebook deriding the greyhound's hopeless arrival. But after a brief, searing run, he popped up again... and squeezed one in. Hope at last.
Even with Henry, Arsenal's top scorer, warming up for Barca, the atmosphere changed. The Emirates was abuzz. A Fabregas free-kick flew high. An equaliser was in the air. Walcott's pace was seriously upsetting the Spanish cool. The magnicently disappointing Messi was absent.
It had to come. Arsenal, habitual scorers of last-gasp goals of late, surged forward. Walcott troubling them. In it comes... and Fabregas goes down under the challenge of Careless Carles Puyol. Penalty from Mr Fussy! Fab steps up and makes it 2-2 with five minutes to play.
He ended up limping to the finish with no subs left... but nearly got on the end of yet another Walcott cross. What a night - what a draw.
Barcelona will return to Spain full of confidence - but at least they'll know they've been in a battle. And for a long time, even that looked unlikely.