It is the ugly side of the beautiful game. This is what officers face at football matches every week.
They've come to watch a game but are instead intent making a scene. It's still an hour to kickoff outside Wembley, but officers are being tested, as are tempers.
When we arrive just a few officers are trying to control a crowd of hundreds of fans, within minutes backup arrives.
Their presence - and fluorescent wall help divide the crowd. Soon one man is arrested but the trouble was much more widespread.
There are 450 officers at Wembley but they have 85,000 supporters to keep safe.
Policing is all about preparation - and they having been planning for this game for months. A team of spotters start the match day with a brief.
It is their job to spot the troublemakers - with the help of video surveillance.
They travel in teams of nine from the station to the stadium - where the match has been given a category B - medium risk.
When they arrive - officers focus on ensuring only one set of supporters are allowed in any single pub.
But at the stadium - it is impossible for police to stop rivals coming into contact. It takes mounted officers to separate and take control of the situation.
The supporters head inside and onto Wembley's land which the stadium has responsibilty for.
By law, the Met Police has a duty to police public land outside grounds - that's costing them £9.3m a season.
But by the time the supporters head home the officers say they and their resources are pretty drained.