Crowd disorder marred Cologne's return to European competition after 25 years as the German team's fans caused havoc in London and forced the Europa League game against Arsenal to be delayed on Thursday.

Arsenal manager Arsene Wenger was surprised that the game was eventually played after a one-hour delay, with his team winning 3-1.

The presence of thousands of ticketless Cologne fans, who marched through the streets of the British capital, made it hard for ticketholders to enter the Emirates Stadium.

Police said four people were arrested for public order offenses. About 20,000 Cologne fans came to London, after only 3,000 tickets were allocated to them.

"At some stage I thought they would not play the game, because I can't see the police taking any risk," Wenger said. "We live in a society of 100 percent security and I thought they would never take a gamble to play this game when I saw the images around the stadium. But I must say our supporters as well dealt well with the situation as well and there was no aggravation."

Wenger said Cologne supporters had been "very clever" to enter the stadium undetected.

"I don't know how they managed to infiltrate our fans and get everywhere but they did that very well," he said. "I don't know if they went through Arsenal membership, on the internet ... they did very well."

Cologne coach Peter Stoger declined to be questioned on the actions of the supporters.

"I have no comment about the fans," he said. "I'm the coach. My job is the team. My job is football not the fans."

Cologne sits winless on the bottom of the Bundesliga, but its fans — walled off by security guards in the stadium were celebrating after only nine minutes when Jhon Cordoba lobbed goalkeeper David Ospina from 40 yards out.

It took Arsenal 40 minutes to reply, through substitute Sead Kolasinac. Alexis Sanchez hit the match-winner, and started the move for the last goal, a rebound effort finished by Hector Bellerin as Arsenal won a Group H match that is likely to lead to UEFA charges over the disorder.

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MILAN START FAST

Milan started quickly in subduing host Austria Vienna 5-1 as forward Andre Silva scored a hat trick.

Midfielder Hakan Calhanoglu opened the scoring after seven minutes then set up two for Silva in the 10th and 20th.

"We played quickly, particularly in the first 20 minutes, finishing the game off clinically," Milan coach Vincenzo Montella said. "This was a big win for us."

Alexandar Borkovic's header reduced the deficit early in the second half, Silva completed his hat trick, and Suso finished the rout just a minute after coming off the bench.

After three years away, AC Milan returned to European competition as perhaps the favorite — at least until December when eight teams jump in as a result of placing third in a Champions League group.

AEK won at Rijeka 2-1 in the Group D game.

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EVERTON STRUGGLES

Atalanta struck three times in the first half to beat struggling Everton 3-0 in Group E on a triumphant return to Europe. Lyon drew with Apollon of Cyprus 1-1 in the group's other game.

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BRAGA RALLIES

Hoffenheim's confidence after beating Bayern Munich in the Bundesliga on Saturday was pricked by Braga, which rallied from a goal down to beat the German host 2-1.

Sandro Wagner headed in a Pavel Kaderabek cross for Hoffenheim to lead in Group C. Joao Carlos equalized, and another Ricardo Esgaio cross set up the match-winner for Dyego Sousa after the break. In the other game in the group, Ludogorets and Istanbul Basaksehir drew 0-0.

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SOCIEDAD SURGE

Real Sociedad scored twice within a minute early in the first half on the way to a 4-0 victory over Rosenborg in Group L. Diego Llorente netted the opener nine minutes into the game, and the closer in the second half. It extended Sociedad's fine start to the season, with the team opening the Spanish league with three wins.

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OTHER GROUPS

In another high-scorer, Alassane Plea scored the first two goals to lead Nice over Zulte Waregem 5-1 in Group K.

Another Italian club, Lazio, came from a 2-1 down to win 3-2 at Dutch side Vitesse in its Group K opener. Murgia scored the winner 15 minutes from time after Ciro Immobile equalized eight minutes earlier.

Marseille beat Turkey's Konyaspor 1-0 in Group I courtesy of a header by Adil Rami from Dimitri Payet's corner kick.