Lionel Messi scored a hat-trick as rampant Barcelona thrashed Celtic 7-0 for their biggest ever Champions League win to kick off the 2016-17 campaign on Tuesday.
Luis Suarez had a brace, and Neymar and Andres Iniesta were also on target at the Nou Camp for Barca who shrugged off a surprise weekend league defeat there against Alaves.
Bayern Munich hammered Russian newcomers Rostov 5-0 for a competition record 13th straight home win, with new German sensation Joshua Kimmich getting a brace.
Visiting Arsenal stole a 1-1 draw at Paris Saint-Germain who wasted a host of chances in a game both sides ended with 10 men.
Only seven games were played Tuesday as the Champions League debut of Pep Guardiola at Manchester City's helm, against Borussia Moenchengladbach, was postponed because of heavy rain in the English city and will now be played Wednesday night.
City are in the same Group C as Guardiola's former club Barca who needed just three minutes to open the scoring against Celtic, Messi firing into the far right corner.
The visitors could have levelled in the 24th when Ter Stegen brought down Dembele but then made amends by saving his weak spot kick.
Messi poked home for 2-0 three minutes later after a fine one-two with Neymar, who then got the third with a curling free-kick in the 50th.
The score was 5-0 10 minutes later as substitute Andres Iniesta volley home Neymar's cross and Messi slide into Suarez' pass to complete his hat-trick. Suarez added the final goals in the 75th and 88th.
"The scoreline is bitterly disappointing," admitted Celtic manager Brendan Rogers, while Barca midfielder Ivan Rakitic tweeted: "Not a bad way to start this season's Champions League."
Bayern dominated possession against the Russian newcomers Rostov but it took a 26th-minute penalty from Robert Lewandowski to break the deadlock.
Thomas Mueller drilled home the second on the stroke of half-time off David Alaba's low cross from the left. Kimmich tapped in for 3-0 in the 53rd and headed home on the hour before Juan Bernat wrapped up matters in the 90th as Carlo Ancelotti enjoyed a winning debut as Munich coach in the competition.
"Things are going well and I want to continue like this," Kimmich, who has also scored in the Bundesliga and for Germany in the past 10 days.
Ancelotti, who has won the Champions League twice with AC Milan and once with Real Madrid, said: "We want to be competitive until the end. The first step is to win the group."
Atletico Madrid, Bayern's conquerors in last season's semis, also won in the other Group D game, 1-0 at PSV Eindhoven. Saul Niguez smashed home the winner in the 42nd as Atletico keeper Jan Oblak then saved a penalty from Andres Guardado.
In Group A, PSG took just 42 seconds to take the lead against Arsenal with newly signed Germany defender Shkodran Mustafi not looking good as the completely unmarked Edison Canavi nodded home Serge Aurier's right-wing cross.
Cavani should have had a hat-trick by half-time as he wasted two other major chances, and Angel di Maria saw his blast saved by David Ospina before the Gunners levelled out of the blue from Alexis Sanchez with 12 minutes left.
Arsenal's Olivier Giroud and Marco Varrazzi of PSG got their marching orders in stoppage time.
The other group game between Basel and Ludogorets also ended 1-1, with Renato Steffen cancelling out the Bulgarians' lead from Jonathan Cafu with 10 minutes left.
In Group B, newly-signed Poliand international Arkadiusz Milik struck twice as visiting Napoli came from behind to beat Dynamo Kiev 2-1. The hosts had led from Denys Garmash but then had Serhiy Sydorchuk sent off for diving twice.
Benfica led Besiktas from Franco Cervi's 12th-minute goal but Talisca curled home a stoppage time equalizer against his own club who had loaned him off to Turkey in summer.
Groups E-H open Wednesday, with holders Real Madrid hosting Sporting Lisbon.