Arsenal’s Europa League campaign began in chaos with their game against Cologne delayed by an hour “in the interests of crowd safety”.
Thousands of ticketless away supporters turned up outside the ground and there were some scuffles between fans and stewards inside after the gates opened.
Police said five people had been arrested on suspicion of public order offences.
There were no reports of trouble during the game, which Arsenal won 3-1.
The German side had only been given a ticket allocation of 2,900.
But around 20,000 of their fans brought central London to a standstill earlier on Thursday afternoon.
At that stage, police said flares were let off but reported “no significant disorder”.
“A group of Cologne fans entered the Emirates Stadium adjacent to the away section and proceeded to fight with stewards in an attempt to gain entry to the away section at around 20:15,” he said.
“A further group arrived around five minutes later and also battled their way into their section.
“Police wearing riot helmets – some with dogs – then entered the stadium and order was restored.”
An Arsenal spokesman said: “The players were relaxed and preparing for the game despite the delay. The decision to go ahead was the best option and the safest option.”
Attempts to keep supporters segregated appeared to have failed with away fans clearly visible in various parts of the stadium.
Cologne are returning to European competition for the first time in 25 years.
Arsenal have not played in the Europa League since 2000. They missed out on Champions League football by finishing fifth in the Premier League last season.
The Metropolitan Police said in a statement: “Officers responded to reports of disorder at the Emirates Stadium during the evening of Thursday, 14 September where a large number of supporters had gathered for the Arsenal vs FC Koln match.
“A flexible policing plan was in place and additional officers were deployed.
“The game has now concluded and the majority of supporters have left the stadium and the surrounding area.
“No reports of any significant incidents.”