City maintained their 100 per cent Premier League start with a 3-0 statement win at rivals Liverpool on the weekend, but Desire Doue in the 13th, Ousmane Dembele two minutes later and Vitinha in the 32nd made it 3-0 at the half on Wednesday (Thursday AEDT).
Erling Haaland headed home for City in the 53rd, Maghnes Akliouche restored PSG's three-goal advantage in the 63rd before Haaland struck again for his 61st goal in the elite event. Ferran Torres missed a late penalty for the visitors.
City were playing their first home match since being found guilty of wide-ranging financial irregularities, and Guardiola showed his support with his first return to Etihad Stadium since the end of his 10-year reign in summer.
Liverpool meanwhile did better when Bradley Barcola's hat-trick helped them shake off their domestic woes with a 6-3 triumph in Austria.
Ronald Araujo had opened the scoring and Jeremy Frimpong was also on target before setting up Barcola's third and the final one from youngster Lewis Koumas. Liverpool become a bit sloppy after leading 6-1, allowing a brace from Florian Flecker after fellow-substitute Robert Ljubicic had earlier netted LASK their first.
"I'm really happy for Bradley because you were asking me yesterday about him not making the numbers, also Florian (Wirtz)," said Liverpool manager Andoni Iraola.
"I think both of them today played really well ... Bradley apart from the hat-trick, I think he was a threat always ... but it wasn't like a big surprise because we were feeling he was in a good place for the game."
Elsewhere, Aston Villa thrashed Fenerbahce 6-0 under the eyes of their big fan, Prince William, who could celebrate a brace each from Nicolas Jackson and Ross Barkley. Johan Manzambi had opened the scoring on his 21st birthday and Tammy Abrahams wrapped up matters.
Jose Mourinho returned with Real Madrid to his former club Roma and claimed an improbable 4-3 triumph from 3-1 down.
The hosts led from Marten de Roon, Kylian Mbappe levelled swiftly for the record champions with a deflected shot that looped in before a second-half brace from Donyell Malen put the Romans 3-1 up.
However, Carlos Espi poked Real's second in the 83rd, Endrick levelled in the 85th and Antonio Rudiger grabbed a 90th-minute winner for Real.
Germany forward Deniz Undav scored his first season goal as visiting Stuttgart won 3-0 at Slovan Bratislava, Bodø/Glimt downed Borussia Dortmund 1-0, Feyenoord defeated Como 3-2, Shakhtar Donetsk beat AEK Athens 3-1 and Real Betis edged FC Porto 3-2.
PSG, Liverpool, Villa and Stuttgart joined Barcelona, Arsenal and Sporting Lisbon on a maximum six points atop the league standings, with Matchday Three following next week.
With PA.
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