In Italy, Serie A leaders Napoli were stunned 3-1 by Bundesliga club RB Leipzig 2-1.
Timo Werner and Bruma’s goals in the 61st and 74th minutes respectively cancelled out Adam Ounas’ 52nd minute opener for Napoli.
Werner then added a third goal for RasenBallsport Leipzig in the 90th minute.
In France, Olympique Lyon saw off Laliga outfit Villareal 3-1.
Lyon shot into the lead in the 46th minute thanks to Tanguy Ndombele’s goal before Nabil Fekir scored on 49 minutes to bring score to 2-1.
In the 63rd minute, Pablo Fornals reduced the deficit for Villareal to make it 2-1.
But with eight minutes left, former Manchester United forward Memphis Depay sealed the game with his goal for Lyon.
And in Scotland, Callum McGregor’s 78 minutes strike saw Celtic pip Zenit St. Petersburg 1-0.
Other Europa League round 32 first-leg results saw Partizan Beograd held 1-1 at home by Viktoria Plzen, FC FCSB edged visiting Lazio 1-0, and Dynamo Kyiv forced home side AEK Athens to a 1-1 draw.
UEFA Europa League round 32 1st-leg results:
Copenhagen 1vs 4 Atletico Madrid
Napoli 1vs 3 RasenBallsport Leipzig
AEK Athens 1 vs 1 Dynamo Kyiv
Celtic 1 vs 0 Zenit
FC FCSB 1 vs 0 Lazio
Lyon 3 vs 1 Villareal
Partizan Beograd 1 vs 1 Viktoria Plzen