Ahead of Sunday's League Cup final against Manchester City, Arsenal had an uncomfortable night against Ostersund. Manager Arsene Wenger made three changes to the team that won 3-0 in Sweden last week with Jack Wilshere captaining Arsenal in the playmaking role instead of Mesut Ozil.
Arsenal's defence looked particularly vulnerable and Ostersund struck twice within a minute in the middle of the first half. Both goals came on fast counterattacks through Hosam Aiesh and Ken Sema who gave Arsenal goalkeeper David Ospina little chance.
One goal away from forcing extra time, Sead Kolasinac came to Arsenal's rescue two minutes into the second half with a goal from close range that reduced Ostersund's lead to the final score of 2-1.
Serie A leader Napoli wasn't good enough to advance in the Europa League on Thursday after it was knocked out in Germany by Leipzig. The last-of-32 round of matches was marred by a police officer's death in Spain after clashes with fans ahead of the game between Athletic Bilbao and Spartak Moscow.
Spanish police said the officer died in hospital after clashes outside San Mames Stadium in the Basque Country city of Bilbao in northern Spain. Spanish media said the officer died of cardiac arrest.
Fans from both clubs were reportedly involved in the clashes.
Heavyweights AC Milan and Arsenal made it through but the London club had to fight hard after an early surprise at home to Ostersund. Arsenal lost 2-1 but advanced 4-2 on aggregate.
In other games, Ciro Immobile powered Lazio to the next round with a hat trick, Atletico Madrid scored only one goal but enjoyed a big 5-1 aggregate win over Copenhagen, while Celtic was eliminated by Russia's Zenit. Borussia Dortmund secured a spot in the next round spot thanks to a late goal by Marcel Schmelzer.
In Bilbao, police were escorting most Spartak fans into the stadium but a stray group started throwing flares and objects toward Athletic fans and police officers.
Spartak won 2-1 in a result that sent the Spaniards through on a 4-3 aggregate.
The draw for the last 16 is scheduled for Friday.
Here's a look at Thursday's games:
In an encounter between two clubs that dropped down from the Champions League, it was Leipzig that made it through at the expense of Napoli despite a 2-0 loss at home on Thursday.
Leipzig advanced on away goals with the aggregate score at 3-3 after winning the first leg 3-1.
Piotr Zielinski put the visitors ahead in the 33rd minute after Leipzig goalkeeper Peter Gulacsi deflected a shot into his path.
Lorenzo Insigne added the second four minutes from time.
Leipzig, currently fifth in the Bundesliga, stunned Napoli at home after coming from a goal down with three late strikes last week.
Kevin Gameiro stretched Atletico's aggregate lead to 5-1 against Copenhagen after an early strike in Madrid to put the result beyond doubt with a 1-0 win.
Atletico has reached two Champions League finals since winning the Europa League in 2012. It returned to the continent's second-tier competition after a disappointing group phase in this season's Champions League.
Lazio rebounded from the first leg 1-0 loss to FCSB, thrashing the Romanian side 5-1 at home to make the next round.
Immobile was unstoppable. He scored twice in the first half and added one more in the second for Lazio to reach 31 goals in all competitions this season. Substitute Bastos and Felipe Anderson added one each.
Milan went through 4-0 on aggregate after beating Bulgaria's Ludogorets 1-0 at home.
Dortmund showed signs of improved form recently, including a 3-2 win over Atalanta in the first leg. On Thursday, Rafael Toloi scored for the Italian side before Schmelzer equalized seven minutes from time for Borussia Dortmund to draw 1-1 and advance 4-3 overall.
Dortmund striker Michy Batshuayi said he was subject to racist chants from Atalanta fans during the game in Italy.
The Belgium international, on loan from Chelsea, tweeted: "2018 and still racists monkey noises in the stands ... really ?!"
Marseille advanced despite a 1-0 loss at Braga after winning 3-0 at home.
Salzburg beat Real Sociedad 2-1, a result that secures a spot in the next round for the Austrian side after drawing 2-2 in the first leg.
Lokomotiv Moscow advanced after eliminating French club Nice 4-2 on aggregate with a 1-0 victory in the second leg in Moscow.
Zenit erased Celtic's first leg 1-0 win with a 3-0 victory after goals from Branislav Ivanovic, Daler Kuzyaev and Aleksandr Kokorin to advance 3-1 on aggregate.
Also, Lyon made it through 4-1 overall after a 1-0 away win at Villarreal.
A 0-0 draw was enough for Dynamo Kyiv against AEK after a 1-1 draw in Athens.
Czech side Plzen beat Partizan Belgrade 2-0 to advance 3-1 on aggregate.
Sporting won 3-1 at Astana last week and advanced after a 3-3 home draw on Thursday.
Associated Press sports writer Tales Azzoni contributed to this report.