Liverpool and Inter both won in added time, while Lionel Messi opened his campaign with a hat-trick.
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- Messi scores hat-trick in 4-0 victory for Barcelona
- Liverpool win in added time after Paris comeback
- Inter score two late goals to beat Tottenham
- Also wins for Atlético, Dortmund and Galatasaray
- Next games in these groups are on 3 October
Monaco 1-2 Atlético Madrid
Samuel Grandsir put Monaco in front, only for Diego Costa and José María Giménez to give the visitors an advantage they would not surrender.
Club Brugge 0-1 Dortmund
This had seemed set to end in stalemate, but Christian Pulišić came off the bench and struck the winner with five minutes remaining – on his 20th birthday, too.
Inter 2-1 Tottenham
Christian Eriksen's deflected first-half effort looked like being enough until Mauro Icardi's thumping volleyed equaliser late on. Matías Vecino then headed Inter's winner two minutes into added time.
Barcelona 4-0 PSV
Messi's superb hat-trick – his record-breaking eighth in the competition – helped the hosts to an emphatic victory. The gloss was only slightly taken off by the second-half dismissal of Samuel Umtiti.
Liverpool 3-2 Paris
Last season's runners-up glided into a 2-0 lead, only for Paris to level through Thomas Meunier and Kylian Mbappé. Liverpool were not to be deterred, though, Roberto Firmino's fine low finish earning maximum points.
Crvena Zvezda 0-0 Napoli
The group stage debutants had a string of fine saves from Milan Borjan to thank for an opening-night point against Carlo Ancelotti's men.
Schalke 1-1 Porto
Breel Embolo's strike midway through the second half put Schalke ahead, but Porto – who had spurned a first-half spot kick – pulled level through Otávio's penalty 15 minutes from time.
Galatasaray 3-0 Lokomotiv Moskva
Garry Rodrigues, Eren Derdiyok and Selçuk İnan – the last with a penalty deep in added time – found the net as Galatasaray opened with a win despite a late red card for Badou Ndiaye.
Wednesday's matches (21:00CET unless stated)
Group E: Ajax v AEK Athens (18.55), Benfica v Bayern München
Group F: Shakhtar Donetsk v Hoffenheim (18.55), Manchester City v Lyon
Group G: Viktoria Plzeň v CSKA Moskva, Real Madrid v Roma
Group H: Valencia v Juventus, Young Boys v Manchester United