Chelsea secured a 2-0 victory in their UEFA Champions League opener against Bayer Leverkusen, while Arsenal conceded a late leveller away to Borussia Dortmund.
All eyes were on Fernando Torres at Stamford Bridge, but it was defender David Luiz and Juan Mata who struck, while substitute Ivan Perisic's late screamer denied the Gunners.
Reigning champions Barcelona fell behind after just 23 seconds against AC Milan at Camp Nou, before recovering to lead 2-1 and then conceding a last-gasp leveller in an entertaining 2-2 draw.
Chelsea boss Andre Villas-Boas made the bold decision to start Torres in their Group E meeting against Bayer Leverkusen, with the Blues striker under scrutiny over a recent interview in which he appeared to criticise some of his team-mates.
The £50million striker again failed to add to his solitary goal for the West London club, but laid off Ashley Cole's pass into the feet of Luiz for the centre-back, who had started the move with a surging run, to steer a sublime strike beyond Bernd Leno.
Villas-Boas, who also left John Terry out of his matchday squad and only afforded Frank Lampard a place on the bench with one eye clearly on Sunday's Premier League showdown with Manchester United, then saw Torres unselfishly set up Mata in the dying moments to secure a 2-0 success.
Arsenal came within minutes of a precious away victory to Borussia Dortmund before super-sub Perisic volleyed home into the top corner from outside the box to cancel out Robin van Persie's opener.
Arsene Wenger's men silenced the vociferous home support with a great goal three minutes before the interval, with the French coach looking on from the stands due to his touchline ban.
Arsenal captain Van Persie slid in to steal a loose ball from Sebastian Kehl before bursting forward and collecting Theo Walcott's precision pass to fire beyond Roman Weidenfeller, only for Perisic to finally find a way beyond Wojciech Szczesny in the 88th minute.
Wenger will be in the stands again for the next meeting with Olympiakos, with the Greek side suffering a 1-0 home defeat to Marseille in their matchday one clash, with Lucho Gonzalez netting the only goal of the game in the 51st minute, while substitute Rod Fanni saw red in the closing moments.
Chelsea's next contest will pit them against Spanish side Valencia, who were held to a 0-0 draw away to Genk in their Group E opener.
AC Milan struck after just 23 seconds and then again at the death to secure a 2-2 draw at Camp Nou against reigning champions Barcelona in Group H.
Pep Guardiola's side were stunned when Alexandre Pato picked the ball up in midfield, burst away from the static home defence and rolled the ball beyond Victor Valdes.
But Pedro levelled from close range after Lionel Messi's superb surging run took him away from the Milan defence to the byline where his cut-back put the chance on a plate.
And David Villa gave Guardiola's men the lead early in the second half with a free-kick of sublime quality which left Christian Abiatti grasping at thin air, only for Thiago Silva to head home an equaliser deep into stoppage time.
Make up the numbers
Viktoria Plzen and BATE Borisov, who are widely expected to make up the numbers in Group H, drew 1-1 in their opening encounter.
Marek Bakos fired the Czech champions ahead on the stroke of half-time after good work from Petr Jiracek before Renan Bressan levelled for the Belarus outfit.
Villas-Boas' former club Porto made a winning start to their Group G campaign as they overcame nine-man Shakhtar Donetsk 2-1 on home soil.
Luiz Adriano had fired the visitors into a surprise lead but Hulk, who had earlier seen his penalty come back off the post, levelled shortly before the half-hour mark.
Yaroslav Rakitskiy, who had conceded the spot-kick, was then dismissed five minutes before interval after felling Joao Moutinho and Kleber put the Europa League winners ahead in the 51st minute after good work from James Rodriguez. Dmytro Chygrynskiy then saw red in the 80th minute.
Apoel Nicosia struck twice in the space of three second half minutes through Jorge Manduca and Ailton to see off 10-man Zenit St Petersburg in Group G's other game.
Konstatin Zyryanov had given the visitors the lead in the 63rd minute, but they imploded soon after by conceding twice and seeing Bruno Alves dismissed for a second bookable offence.
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