AC Milan withstood a late onslaught to beat Celtic 3-1 and AZ Alkmaar overcame the loss of 13 players with positive COVID-19 tests to secure a shock 1-0 win at Napoli as the Europa League group stage got underway on Thursday.
London sides Tottenham Hotspur and Arsenal also made winning starts as did fellow Premier League club Leicester City.
Dutch team Alkmaar were under pressure for the whole game as Napoli dominated possession and peppered their goal but Dani de Wit scored in the 57th minute for a stunning victory in Group F.
With Milan's defence well marshalled by Danish centre-back Simon Kjaer, they looked to be cruising to an easy win at Celtic after first-half goals from Rade Krunic and Brahim Diaz.
However, the Scottish champions came roaring back in the second half and reduced the deficit through Mohamed Elyounoussi. Milan needed a late goal from Jens Petter Hauge, his first for the club, to put the Group H game out of reach.
Celtic's disappointment was summed up by midfielder Ryan Christie. "I'm gutted to lose tonight, I did feel like we could have pushed forward and got an equaliser, but there's positives to take from the second half," he said.
Villarreal and Turkey's Sivasspor played out an eight-goal thriller in Group I, with the Spanish side grabbing a 5-3 win thanks to a late double from striker Paco Alcacer.
Arsenal, who have not claimed a European trophy since lifting the old Cup Winners' Cup in 1994, went behind at Rapid Vienna but defender David Luiz and striker Pierre-Emerick Aubameyang struck late on for a 2-1 Group B win.
Fellow Premier League sides Leicester City and Tottenham Hotspur cruised to comfortable 3-0 wins, with the Foxes downing visiting Ukrainian side FK Zorya Luhansk in Group G, while hosts Spurs beat Austrians LASK in Group J.
Tottenham's fine performance, including an assist from Gareth Bale on his first start back at the club on loan from Real Madrid, spurred on the London side who lost the 2019 Champions League final to Liverpool.
""We went out there today and did a great job. We were solid and threatening and the most important thing in these games at the start is to get the three points," Bale told BT Sport.
Bayer Leverkusen went on the rampage against French side Nice in a 6-2 victory that saw winger Karim Bellarabi score twice late on for the home side in Group C.
AS Roma came back from a goal down to beat Swiss side Young Boys 2-1 away thanks to two goals in five second-half minutes by Bruno Peres and Marash Kumbulla in Group A.
Spanish clubs Granada and Real Sociedad enjoyed 2-1 and 1-0 away wins over PSV Eindhoven and Croatian side Rijeka in Group E and F respectively, while Benfica chalked up a 4-2 victory over Lech Poznan and Rangers won 2-0 at Standard Liege in Group D.