Barcelona opened their Champions League campaign with a 2-1 defeat in Monaco after playing most of the match with 10 men following an early red card for defender Eric Garcia.
Hansi Flick has made an impressive start at Barcelona after taking over in May, winning their opening five LaLiga fixtures to sit top of the table.
However, the Spanish side found themselves down to 10 men after just 10 minutes of their first European fixture when Garcia was shown a straight red card for hauling down Takumi Minamino.
Maghnes Akliouche then put Monaco ahead, only for 17-year-old Lamine Yamal to drill in a well-crafted equaliser.
Monaco, though, regained the lead with 20 minutes left through substitute George Ilenikhena.
It could have been worse for Barcelona when Monaco were initially awarded a late penalty, only for the Dutch referee to overturn his decision after a review.
Bayer Leverkusen had earlier marked their return to Europe’s top club competition with a 4-0 win at Feyenoord.
Florian Wirtz gave the Bundesliga champions an early lead at De Kuip and Alex Grimaldo doubled the lead on the half-hour, with former Celtic man Jeremie Frimpong soon setting up Germany international Wirtz for his second.
Feyenoord saw their forgettable night complete when goalkeeper Timon Wellenreuther spilled a header from Edmond Tapsoba into his own net just before half-time.
Jose Maria Gimenez’s last-minute goal saw Atletico Madrid come from behind to beat RB Leipzig 2-1 at the Metropolitano Stadium.
RB Leipzig had taken an early lead through Benjamin Sesko’s header. Atletico, though, were level in the 28th minute when Antoine Griezmann crashed home a volley.
Just when it seemed both sides would be settling for a draw, Griezmann’s cross picked out Gimenez to head in a dramatic winner – sparking wild celebrations from the Atletico bench.
Arsenal battled to a goalless draw away at Atalanta, where goalkeeper David Raya pulled off a brilliant double save in the second half.
Atalanta were awarded a penalty in the 51st minute for a foul by Thomas Partey on Ederson.
Raya got down to his right to save Mateo Retegui’s spot-kick – and then leapt up to palm away his header from the rebound to help earn his side a point.
Brest made a winning start on their Champions League debut after they beat Sturm Graz 2-1.