Last season's beaten finalists Liverpool got their Champions League campaign off to a disastrous start in a 4-1 defeat away to Napoli on Wednesday, while Robert Lewandowski hit a hat-trick in a big win for Barcelona.
Tottenham Hotspur and Bayern Munich also got their campaigns underway with victories, but Rangers were crushed 4-0 by Ajax in the Netherlands on their return to the group phase of Europe's elite club competition.
Jurgen Klopp's Liverpool have won just two of their first six Premier League games this season and there was to be no improvement on their trip to Italy as Napoli raced into an early lead and never looked back.
Piotr Zielinski converted a fifth-minute penalty, and Victor Osimhen then had a second Napoli spot-kick saved by Alisson Becker.
Yet Napoli -- who had defeated Liverpool in their own stadium in the group stage in 2018 and in 2019 -- were 3-0 up at the break as Andre-Frank Zambo Anguissa's fine finish was followed by a tap-in from substitute Giovanni Simeone.
Liverpool fell further behind two minutes into the second half as Zielinski followed up to score after his initial effort was saved, but Luis Diaz did pull a goal back.
"We were miles too open. You can't come to a place like this and not be compact. They were by miles the better team," Liverpool left-back Andy Robertson told BT Sport.
"We have to wake up and quickly because we can't perform like that."
- Rangers crushed by Ajax -
The six-time European Cup winners' next Group A outing is at home to Ajax, who could have won by more than four at home to Rangers, last season's Europa League finalists playing in the Champions League group stage for the first time since 2010.
Edson Alvarez headed Ajax in front before a Steven Berghuis shot deflected in and a superb individual strike from Mohammed Kudus made it 3-0 to Ajax by half-time.
Steven Bergwijn pounced on a poor backpass to seal the victory late on in Amsterdam, leaving Rangers manager Giovanni van Bronckhorst to give a bleak assessment of his side's prospects.
"To compete in the Champions League you spend hundreds of millions, for us to compete it is too much to ask," insisted the Dutchman.
Lewandowski became the first player to score a Champions League hat-trick for three different clubs as the ex-Borussia Dortmund and Bayern star marked his first appearance in the competition with Barcelona in style.
The Catalans crushed Czech champions Viktoria Plzen 5-1 in Group C, with Franck Kessie heading in the opener before Lewandowski made it 2-0.
Jan Sykora pulled one back only for Lewandowski to score again before half-time and then fire in his third goal midway through the second half.
The Pole now has 89 Champions League goals, three more than Karim Benzema, with only Cristiano Ronaldo and Lionel Messi ahead of him.
Ferran Torres scored the fifth for Barca, whose next game will be away to Lewandowski's old side Bayern.
The Germans claimed an impressive 2-0 win away to Inter Milan on Wednesday, with Leroy Sane rounding Andre Onana for the first-half opener and then forcing Danilo D'Ambrosio into a 66th-minute own goal.
- Richarlison brace for Spurs -
Antonio Conte's Tottenham struggled to break down Marseille in north London until the French side had Chancel Mbemba sent off just after half-time.
Richarlison scored the opener in the 76th minute, heading in Ivan Perisic's cross, and the Brazilian headed in again soon after to make it 2-0.
"I think we felt a bit of pressure in this competition, but at the end we have to be pleased," Conte said of the victory.
Also in Group D, Sporting Lisbon scored three second-half goals to win 3-0 at Eintracht Frankfurt, with former Spurs youngster Marcus Edwards, Francisco Trincao and Nuno Santos all on target.
In Group B, Antoine Griezmann headed the winner in the 11th minute of injury time as Atletico Madrid beat Porto 2-1.
In an incredible finish, Porto had Mehdi Taremi sent off late on before Mario Hermoso put Atletico ahead with a deflected 92nd-minute strike.
Hermoso then conceded a penalty for handball, allowing Matheus Uribe to equalise in the 96th minute, only for Griezmann to have the last laugh.
The only other game in Champions League history to feature three goals scored in stoppage time was Porto's meeting with Atletico in December last year.
In the same group Abakar Sylla's header was spilled over the line by the goalkeeper to give Club Brugge a 1-0 win over Bayer Leverkusen.