Alexis Sanchez scored in the second half to help Arsenal salvage a 1-1 draw at Paris Saint-Germain, who were left to rue multiple missed chances after scoring in the first minute of their Champions League opener.
It took only 42 seconds for Edinson Cavani to head the French champions in front, but the Uruguayan missed three more clear opportunities before Sanchez's 77th-minute equaliser.
Mesut Ozil played a pass into the box to an open Alex Iwobi, whose hard shot -- Arsenal's first on target of the game -- was saved by Alphonse Areola, but it came right back out for Sanchez to score inside the near post.
There would be no more goals, but there was some late drama as Giroud and Marco Verratti came together in stoppage time and both received their second yellow cards -- ruling them out of Matchday 2.
Barcelona opened their group stage campaign in fine fashion as Lionel Messi scored a hat trick and Neymar assisted on four goals in a 7-0 thrashing of Celtic at the Camp Nou.
Messi opened the scoring for the hosts in the third minute, but Celtic then wasted a dream chance to equalise when Moussa Dembele saw a penalty saved by Barca goalkeeper Marc Andre ter-Stegen.
Minutes later, Messi toe-poked home for his second goal and it was 3-0 less than five minutes after the break, as Neymar curled a free kick over the Celtic wall and past De Vries from 20 yards out.
Next up for Barcelona was Andres Iniesta, who came on for Ivan Rakitic at half time, and the Spain international made no doubt when he smashed a volley home from 10 yards away following a cross by Neymar to make it 4-0.
Messi had his hat trick a minute later to make it 5-0, as he tapped home a pass from Suarez from close range before the Uruguayan added two late strikes to seal a memorable victory.
The other game in Group C, Manchester City's clash with Borussia Monchengladbach, will be played on Wednesday after it was postponed because of torrential rain.
Midfielder Joshua Kimmich continued his goal rush as he claimed a double in Bayern Munich's 5-0 Champions League demolition of FC Rostov.
The 21-year-old took his tally for club and country to four goals in three games, having only scored his first for his club against Schalke on Friday, as Carlo Ancelotti's men romped to a record 13th successive home victory in the competition.
Robert Lewandowski opened the scoring from the penalty spot and Thomas Muller extended the lead in added-time at the end of the first half, but it was Kimmich who wrapped up the first three points of his side's Group D campaign with a brace inside seven second-half minutes before substitute Juan Bernat added a fifth at the death.
Elsewhere in Group D, Atletico Madrid's bid to reach a third Champions League final in four years got off to a winning start with a hard-fought 1-0 victory over PSV Eindhoven.
Saul Niguez's acrobatic strike just before half-time settled an eventful encounter at the Philips Stadion as Atletico claimed a valuable away win.
PSV had the chance to net an immediate equaliser after winning a hugely controversial 45th-minute penalty, but Andres Guardado saw his spot-kick superbly saved by Jan Oblak.
Arkadiusz Milik was Napoli's hero as they came from behind to beat 10-man Dynamo Kiev 2-1 in Ukraine.
Milik's headed first-half double was enough to seal a precious Champions League Group B away win and set up the Italians perfectly for their home clash with Benfica in a fortnight's time.
Denys Garmash had put the home side ahead, but Milik's quick-fire contribution proved decisive, particularly after Dynamo midfielder Serhiy Sydorchuk was dismissed for a second bookable offence.
On-loan midfielder Anderson Talisca scored a stoppage-time free-kick against parent club Benfica to earn Besiktas a 1-1 draw on their return to the Champions League after a six-year absence.
Benfica, who reached the quarterfinals last season where they lost to Bayern Munich, looked on course to start their Group B campaign with all three points following a first-half goal from Franco Cervi.
However, the Turkish side produced a late rally and secured what looked an unlikely draw two minutes into stoppage time when Talisca -- who was allowed to leave the Portuguese side last month -- whipped a 25-yard free-kick up over the wall and into the top left corner.
Basel fought back to secure a 1-1 draw in their Champions League Group A opener against Ludogorets at St Jakob-Park.
Jonathan Cafu fired the Bulgarians ahead on the stroke of half-time, but Basel, who have won eight games from eight so far this season and are already nine points clear of nearest rivals Young Boys in the Swiss top flight, produced a strong second-half showing and were rewarded with 10 minutes left when Renato Steffen hooked the ball in at the far post.
With these two sides expected to contest third place behind experienced European campaigners PSG and Arsenal, neither could really afford to lose early ground with defeat in the opening group games.