Arsenal recovered from a goal down to beat FC Cologne 3-1 after crowd disturbances in London, while AC Milan's new signing Andre Silva scored a hat trick in a rousing 5-1 win at Austria Vienna as the Europa League group stage got underway on Thursday night.
Fellow Serie A sides Atalanta and Lazio also won, while Portugal's Vitoria Guimaraes began their match against Salzburg with not a single European player on the team.
Arsenal's Group H match was delayed by an hour as about 20,000 Cologne supporters fans, many without tickets, descended on Emirates Stadium before kickoff as their team returned to European competition for the first time in 25 years.
There were also scuffles between fans and when the action finally started, Arsenal fell behind to a stunning long-range strike from Jhon Cordoba before second-half goals from Sead Kolasinac, Alexis Sanchez and Hector Bellerin rescued the win.
Milan's Portugal forward, signed from Porto in the offseason, enjoyed a prolific night as he became the first player to score a European hat trick for the seven-time European champions since Brazilian Kaka against Anderlecht in 2006.
Hakan Calhanoglu set Milan on its way with a fierce drive in the seventh minute and then set up Silva's first two goals in the 10th and 20th of the Group D game.
Although Alexandar Borkovic pulled one back two minutes after the break, Silva completed his treble in the 56th minute after being sent clear by Franck Kessie's through ball. Suso added the fifth just after the hour before Milan eased up.