"Officers are dealing with disorder at the Emirates Stadium where fans have gathered for the Arsenal vs FC Koln match", said a pre-match statement issued by London's Metropolitan Police.
Arsene Wenger has opted to make several changes from the side which beat Bournemouth comfortably at the weekend as the Chilean global is afforded his first start at the Emirates Stadium this season.
The German club are returning to European competition for the first time in a quarter of a century.
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.
But substitute Sead Kolasinac equalised with a thumping volley before Sanchez struck, and Hector Bellerin then added the third from close range, as midfielder Jack Wilshere made his first appearance for the club since August 2016.
"That they outsung Arsenal didn't really surprise people though - Arsenal isn't exactly known as a club that creates a particularly tingling atmosphere". In the new season, Arsenal has not been very impressive with its game, having won 2 wins with 2 defeats for 4 rounds of the Premier League.
"Was it the fact that they scored the goal and we didn't know how to deal with it?"
"In the second half he came alive".
"I believe our team was better balanced, better equipped going forward". The Gunners must get back into their winning form, but Arsene Wenger doesn't seem to think that the Europa League is the best way for his squad to get back in the action. You have to be ready, these things happen - not often, but you have to be ready.
Full-back Hector Bellerin put the result beyond doubt nine minutes from the end and the Spain global added: "It is a new competition for us so we had to set the tone". We have shown good spirit, good football and good wins most importantly of all'.