Thousands of German football fans brought parts of central London to a standstill ahead of their team's Europa League match against Arsenal.
Some 20,000 FC Cologne fans have come to London, and briefly brought the West End traffic to a halt as they marched down Oxford Street.
The game was delayed by an hour, which Arsenal said was "in the interests of crowd safety".
Police said they were dealing with disorder at the Emirates Stadium.
More than a dozen police officers wearing helmets could be seen entering the ground as the game kicked off and riot police could be seen sitting outside the stadium.
It is the first time in 25 years Cologne has played in an international club competition.
The Metropolitan Police said in a statement: "Officers are dealing with disorder at the Emirates Stadium where fans have gathered for the Arsenal v FC Cologne match.
"A policing plan is in place for the game and additional officers have been deployed."
Earlier, police said flares were let off and bottles thrown but reported no "significant disorder".
The Bundesliga estimated around 20,000 fans had travelled to the capital to support their team.
It added only 2,900 tickets had been allocated to the away fans for the match.
Earlier, the Met said: "At around 15:00 BST on Thursday, 14 September a large group of football fans gathered in Oxford Street.
"The group was at the location for about 30 minutes whilst they boarded public transport to travel to a football match this evening.
"They did throw bottles and let off flares, but there was no significant disorder, police were on scene and there have been no arrests."