Geoffrey Kondogbia’s deflected 25-yard strike gave Monaco the lead seven minutes before half-time in north London.
Former Tottenham Hotspur striker Dimitar Berbatov then finished off a counter-attack from 12 yards to make it 2-0 in the 53rd minute.
Arsenal pulled a goal back when substitute Alex Oxlade-Chamberlain curled home in the 90th minute.
But Yannick Ferreira Carrasco scored in stoppage time to put Monaco in charge of the last-16 tie ahead of the second leg on 17 March.
The game prompted plenty of activity on Twitter – and here are some of our favourite updates.
Gary Lineker (@GaryLineker) is not impressed by the Gunners’ display as Monaco take the lead: “It’s extraordinary how many times Arsenal leave themselves so exposed to that kind of counter attack. Beyond amateurish!”
Jamie Carragher (@Carra23) isn’t happy: “Arsenal frustrate me so much & I’m not even a fan!”
Jan Aage Fjortoft (@JanAageFjortoft) can’t believe what he has seen from Per Mertesacker: “Mertesacker’s action ahead of that 2-0 is…..astonishing”
Lord Sugar (@Lord_Sugar) is enjoying himself: “Ex Spurs player makes it 0-2 Arsenal v Monaco how you feeling @piersmorgan @IanJamesPoulter”
Phil Neville (@fizzer18) is not impressed by Arsene Wenger’s decision to substitute Olivier Giroud: “Don’t take off your centre forward, when Walcott crosses Giroud needs to be in the box!! Arsenal need all goals on pitch!! Ozil off for me!!”
Celebrity Arsenal fan Piers Morgan (@piersmorgan) needs some consoling: “Once again, it’s the hope that kills. #Arsenal”
Carragher (@Carra23) chimes in again after the final whistle: “Monaco have been brilliant proper European performance, organised, resolute in defence & excellent on the counter attack.”
And that just leaves Lee Dixon (@LeeDixon2) to have the final say: “Not one aspect of Arsenal’s game worked tonight. Felt sorry for Alexis. Effort for 90 mins. Everyone else. Poor. Monaco suprised me. Good”
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