AFP, PARIS: Alexis Sanchez scored a stunning goal as Arsenal came from behind to beat Cologne 3-1 in a Europa League clash on Thursday that was delayed for an hour due to crowd trouble.

Kick-off in the Group H opener at the Emirates Stadium was put back until 9:05pm local time (2005 GMT) in the interests of "crowd safety" after problems caused by thousands of ticketless Cologne supporters arriving at the ground.

Police confirmed four arrests were made before kick-off while there were reports of scuffles inside the stadium.

However, when the match did get under way, those travelling fans who got in were treated to a stunning 40-yard strike by Jhon Cordoba that put Cologne ahead with Arsenal goalkeeper David Ospina caught out of his area.

Arsenal improved in the second half and substitute Sead Kolasinac volleyed them level before a brilliant goal by Sanchez gave them a 67th-minute lead.

The Chilean cut in from the left flank before curling a sublime effort into the far top corner, and the three points were secure when Hector Bellerin made it 3-1 late on after Theo Walcott's effort had been saved by Timo Horn.

“The win, and coming back from losing, obviously gives you a boost. Now we'll train hard in the coming days to be ready for the weekend,” Bellerin, whose side go to Chelsea on Sunday, told BT Sport.

Arsene Wenger's side are next in European action in a fortnight in Belarus against BATE Borisov, who drew 1-1 at Red Star Belgrade.

Meanwhile, Arsenal will investigate how vast numbers of Cologne supporters gained access to the home sections of the Emirates Stadium for Thursday's Europa League match, sparking crowd trouble.

Visiting fans were issued with 3,000 tickets but around 20,000 fans of the German club are believed to have made the journey to London.

There were clearly thousands of away supporters sitting among Arsenal fans, causing a security risk.

Five arrests were made and Arsenal issued a statement on Friday insisting the safety of supporters was the main concern at the time.

"Following last night's UEFA Europa League match with Cologne we would like to stress that fan safety was always our paramount concern and informed all decisions made," it said.

“We have launched a full review into the circumstances surrounding the game and will ensure any lessons that can be learned are used in the future.”

A UEFA probe is also likely after pictures emerged of a minority of Cologne supporters causing problems for stewards inside the stadium.