Napoli started their Europa League campaign with a shock defeat at home to AZ Alkmaar, while Arsenal's blushes were spared by goals from David Luiz and Pierre-Emerick Aubameyang in their come-from-behind win in Rapid Vienna.
Alkmaar had travelled to Italy with 13 of their players sidelined because of coronavirus, but the Dutch runners-up were still able to spring a surprise against a side that reached the last 16 of the Champions League last season.
Dani De Wit scored the decisive goal at the Stadio San Paolo, finding space in the area before drilling beyond Napoli goalkeeper Alex Meret from close range after 57 minutes, as Alkmaar also prevailed 1-0 in Group F, where Real Sociedad won at Croatian club Rijeka following Jon Bautista's strike in time added-on.
A mistake by Arsenal goalkeeper Bernd Leno gifted a goal to Taxiarchis Fountas as Rapid Vienna went into a 51st-minute lead but the Gunners hit back to win 2-1 at the Allianz Stadion, where 3,000 home fans watched on.
Arsenal scored twice in four minutes with Luiz levelling and substitute Aubameyang bagging the winner a quarter of an hour before full-time in Group B, where Norwegian champions Molde also won 2-1 after coming from behind to beat Irish side Dundalk.
Gareth Bale made the first start of his second spell with Tottenham, who breezed to a 3-0 Group J triumph over LASK thanks to strikes from Lucas Moura and Heung-Min Son either side of an own goal from Andres Andrade. Lior Refaelov scored the winner for Royal Antwerp, who saw off Ludogorets 2-1 in the other group game.
Rade Krunic, Brahim Diaz and Jens Hauge were all on the scoresheet as AC Milan prevailed 3-1 at Celtic, who scored a consolation through Mohamed Elyounoussi, while also in Group H a hat-trick from Yusuf Yazici lifted Lille to a 4-1 victory over 10-man Sparta Prague.
James Tavernier's 19th-minute penalty and substitute Kemar Roofe's halfway line goal in time added on lifted Rangers to a 2-0 win at a rain-lashed Standard Liege in Group D, while Uruguayan striker Darwin Nunez's hat-trick propelled Benfica to a 4-2 victory at Lech Poznan.
James Maddison, Harvey Barnes and Kelechi Iheanacho were all on target as Leicester showed no signs of missing the injured Jamie Vardy in a 3-0 win over Ukrainian visitors Zorya Luhansk in Group G, where Braga brushed aside AEK Athens by the same scoreline.
Bayer Leverkusen hammered Nice 6-2 in Group C, where Hapoel Be'er Sheva overcame Slavia Prague 3-1, while Granada defeated PSV Eindhoven 2-1 in Group E as PAOK Salonika drew 1-1 against Omonia Nicosia.
Roma came from a goal down to claim a 2-1 victory at Young Boys while also in Group A CFR Cluj ran out 2-0 winners at CSKA Sofia. In Group K there were draws between Dinamo Zagreb and Feyenoord and Wolfsberg and CSKA Moscow.
Paco Alcacer's double helped Villarreal defeat Sivasspor 5-3 in a Group I thriller, while Yonatan Cohen's early strike gave Maccabi Tel Aviv a 1-0 victory over Qarabag. In Group L, Slovan Liberec and Hoffenheim each made winning starts after seeing off Gent and Red Star Belgrade respectively.