Champions League holders Real Madrid earned a last-gasp 2-1 victory over Sporting Lisbon in their opening Group F match at the Bernabeu thanks to late goals from Cristiano Ronaldo and Alvaro Morata.
Bruno Cesar's goal in the 47th minute gave the visitors the lead over the defending champions, but an 89th minute free kick from Ronaldo and a stoppage time header from Morata secured the late win.
Rampant Leicester City marked their Champions League debut in stunning style as they thrashed Club Brugge 3-0.
Boss Claudio Ranieri had warned against expecting another fairytale but it was dream night in Belgium after Leicester scored twice inside the first half an hour.
Marc Albrighton netted the Foxes' first-ever Champions League goal, and first in Europe since 2000, before Riyad Mahrez crashed in a brilliant free-kick.
Mahrez made it 3-0 with a second-half penalty with Leicester, who dominated at the Jan Breydel Stadium, top of Group G after Porto and Copenhagen drew 1-1.
Manchester City kept up their perfect start to the season and began their Champions League campaign with a win, at the second attempt, by beating Borussia Monchengladbach 4-0 at the Etihad Stadium courtesy of a Sergio Aguero hat trick.
Aguero turned in a low cross from Aleksandar Kolarov after eight minutes, added a 28th-minute penalty when debutant Ilkay Gundogan was fouled, and completed his treble on 77 minutes when Raheem Sterling's through-ball allowed him to round Gladbach goalkeeper Yann Sommer and tap in.
Kelechi Iheanacho came off the bench to add a late fourth. That result puts City second in Group C, behind Barcelona on goal difference.
It wasn't a good start for Tottenham Hotspur at Wembley on their Champions League return, however, as they crashed to a 2-1 defeat against AS Monaco.
Bernardo Silva opened the scoring for Monaco in the 15th minute before Thomas Lemar doubled the Ligue 1 side's lead in the 31st minute.
Toby Alderweireld rose to head home an Erik Lamela corner kick at the near post to give them hope at the break but Spurs were unable to complete the turnaround.
Juventus were frustrated by a stubborn Sevilla side in Turin as the 2015 Champions League finalists started this year's campaign with a 0-0 draw.
Without the recently departed Alvaro Morata and Paul Pogba, two key figures in their run to the final in Berlin two seasons ago, Juve could not unlock the Spanish team that has won the previous three Europa League titles, the most recent of which ensured they went straight into the group stages this term.
Record signing Gonzalo Higuain came closest when he hit the crossbar, while Sami Khedira was wasteful with two clear openings in the first period.
Borussia Dortmund demolished Legia Warsaw in Poland as half-a-dozen different players found the net in a 6-0 Champions League rout.
Three goals in the opening 17 minutes from Mario Gotze, Sokratis Papastathopoulos and Marc Bartra saw Thomas Tuchel's side make the most of some shambolic defending to take charge.
Raphael Guerreiro then fired in a fourth, substitute Gonzalo Castro added a fifth with his first touch and then Pierre-Emerick Aubameyang got in on the act to round off a comfortable night for the Germans.
Alan Dzagoev's superb 36th-minute strike sparked a fine comeback as CSKA Moscow grabbed a 2-2 draw with Bayer Leverkusen in their Champions League Group E opener in Germany.
The Russians had been reeling when Admir Mehmedi and Hakan Calhanoglu struck in the opening 15 minutes to give the hosts a magnificent start.
But after Dzagoev rifled his opener past Leverkusen keeper Bernd Leno, teammate Roman Eremenko followed up two minutes later to ensure the visitors were back on level terms before half-time.
Andreas Cornelius netted a second-half equaliser as 10-man FC Copenhagen fought hard to earn a 1-1 draw at Porto in Champions League Group G.
The former Cardiff striker netted from close range shortly after the interval to cancel out Otavio Monteiro's fine early opener for the hosts.
Copenhagen were then forced to hold on for a point despite having to play the final 25 minutes with 10 men after Jan Gregus was harshly sent off for a second yellow card.
Lyon eased the pressure on coach Bruno Genesio with a comfortable 3-0 win over Dinamo Zagreb to open their Champions League campaign.
Corentin Tolisso's first-half opener was followed by goals from Jordan Ferri and Gnarly Cornet after the break to end a two-game losing run that had raised some questions about Genesio's future.