Alexis Sanchez scored a stunning goal as Arsenal came from behind to beat Cologne in a Europa League match delayed for an hour following crowd trouble.
Chilean Sanchez, who nearly left the club to join Manchester City on transfer deadline day, picked the ball up outside the area and curled a shot past keeper Timo Horn.
The game did not begin until 21:05 BST as thousands of visiting supporters arrived at the ground without tickets andthen clashed with stewards inside the Emirates Stadium.
When the match did get under way, Cologne took the lead in spectacular fashion as Jhon Cordoba lobbed fellow Colombian David Ospina from 40 yards.
Striker Olivier Giroud planted a header wide of goal from six yards for the much-changed home side, who were booed at the half-time whistle.
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.
In Group H’s other game, Red Star Belgrade drew 1-1 against BATE Borisov.
How FC Cologne fans (briefly) took over London
The importance of Alexis
Forward Sanchez scored 30 goals in all competitions for Arsenal last season as they won the FA Cup.
But in August, he almost joined former boss Pep Guardiola at Manchester City for £60m, but the Gunners pulled out of the deal after failing to find a replacement.
Sanchez made his second start of the season in an impressive showing, capping his performance with a sublime effort in the second half – his first goal of the season against the Bundesliga’s bottom side.
He could have scored another two, but struck a free-kick straight at Horn and shanked wide from inside the area.
Forgotten man Wilshere, who spent last season on loan at Bournemouth before suffering a broken leg, came on for Alex Iwobi on 68 minutes, and the England international’s clever dummy was instrumental in the lead-up to Bellerin’s goal.
20,000 does not fit into 2,900
Cologne fans had been given a ticket allocation of 2,900 but 20,000 fans travelled over from Germany
Cologne, like Arsenal, finished fifth in their domestic league last season, achieving European football for the first time in 25 years.
The Bundesliga club’s return to European competition, though, was one tainted by controversy.
There were indications on Thursday afternoon that problems may occur, with more than 20,000 fans arriving from Germany, despite the visitors receiving an allocation of 2,900 tickets.
Videos emerged on social media of the German side’s support briefly bringing parts of central London to a standstill as they threw bottles and let off flares while making their way to the ground.
The kick-off was then put back, after which skirmishes between fans and stewards took place, while many had entered the ground into the home end, climbing barriers to get into the away section.
A number of Arsenal fans inside texted BBC Football, with one supporter reporting that they felt “intimidated by the horrible, tense atmosphere”, another that they were “ashamed of Arsenal for not seeing the warning signs” and a third describing it as “the worst feeling at football in 40 years of watching”.
It remains to be seen whether Uefa charges are brought against one or both sides.
Thursday night fright
Jhon Cordoba (right) scored from around 40-yards out
Arsenal had finished in the Premier League’s top four in each of Arsene Wenger’s 20 seasons in charge of the club until last term.
May’s fifth-place finish meant missing out of Europe’s elite club competition and an entry into the secondary tier, where they have not played since 2000.
On that occasion, after finishing third in their Champions League group, they made it all the way to the Uefa Cup final before being beaten by Turkish side Galatasaray on penalties in Copenhagen.
French boss Wenger said finishing in the Premier League’s top four was their best route of getting back into the Champions League and so he left out many of his first-team players for the game against Cologne, instead giving those on the fringes an opportunity.
But goalkeeper Ospina suffered a dreadful start, rushing off his line and failing to get distance on a clearance which fell to the feet of Leonardo Bittencourt, who laid it off to Cordoba.
The striker took a touch, spun and curled a sublime, long-range effort over the head of Ospina and into the net.
Petr Cech’s understudy made up for his error by saving well off Cordoba low down in the second half and collecting a vicious drive from Milos Jojic as his side turned the game around.
Man of the match – Alexis Sanchez (Arsenal)
Sanchez scored his first goal in the Europa League… and what a goal it was
No hierarchy in the squad – Wenger
Arsenal manager Arsene Wenger, speaking to BT Sport: “We had a slow start and there was accidental goal. After that, it was important not to concede and panic and put more pace into our game. From then. we controlled the game and won convincingly in the end.
“I think Sanchez is still running after his best form. He is a fantastic football player but physically he has just come back. But he has it in his locker to do something special, which he did.
“We want to do well in all the best competitions and I had an experienced team today, even though I made nine changes. They are all top-class players. There is no hierarchy in the squad.”
Arsenal full-back Hector Bellerin: “It is a new competition for us so we had to set the tone. We think we are one of the best teams in the competition so we have to show that on the pitch.
“I am very happy with the victory. We were ready to play at 20:05 BST but these things happen. We started a bit with cold feet but after that it just worked.”
Arsenal face London rivals and Premier League champions Chelsea on Sunday (kick-off 13:30 BST), while Cologne will look to pick up their first points in the Bundesliga when they travel to Borussia Dortmund on the same day (kick-off 17:00 BST).
Classy Kolasinac – the stats
This was Arsenal’s first European match outside of the Champions League (including qualifiers) since May 2000 – 17 years and 120 days ago.
Cologne lost a competitive game that they were winning at half-time for the first time since December 2014 (1-2 v Augsburg).
Arsene Wenger has registered a win over 57 different clubs during his tenure as Arsenal manager (from 64 teams faced).
Alexis Sanchez has scored seven goals in his last eight competitive appearances for Arsenal.
Hector Bellerin scored his first European goal in his 16th appearance in European competition.
Sead Kolasinac has played a part in four goals (two goals, two assists) in five appearances for Arsenal (including Community Shield).
Kolasinac’s goal was the 200th scored at home in European competition under Wenger – 125 scored at the Emirates, 69 at Highbury and eight at Wembley.