The visitors controlled a cagey first period, which culminated in Gyokeres blasting home after he was clumsily bundled over in the area by David Hancko. David Raya had been forced into a good save from a fine Alvarez strike and Noni Madueke went close at the other end with a curling effort, but otherwise it was a half of few chances.
However, it was a different story after the break as Arsenal relinquished that control and found themselves clinging on at times. Alvarez levelled with a penalty of his own 10 minutes into the second 45 after Ben White was punished for a handball. That preceded a flurry of close calls for the Gunners, most notably Raya's close-range stop from Ademola Lookman, Gabriel Magalhaes' incredible block to deny Antoine Griezmann and the Frenchman hitting the woodwork.
Arsenal thought they had the third spot-kick of the night and a chance to win the game with just 10 minutes to play, as substitute Eberechi Eze went down under another Hancko challenge - but the referee controversially reversed his decision after consulting the pitch-side monitor. Their frustration was almost compounded in added time, but thankfully Nahuel Molina's drive flew just over as the Gunners were made to settle for a creditable first-leg draw.
GOAL rates Arsenal's players from the Metropolitano...
