Russia and Poland played out a 1-1 draw in perhaps the most feared match of Euro 2012 from a security standpoint. The two countries have a long history of military invasions and occupations and supporters from both sides seemed intent on continuing those hostile relations. Prior to the match (which happened to occur on Russia Day -- Russia's independence day), police fired rubber bullets and fans from both sides were involved in various fights around Warsaw, resulting over 180 arrests and 24 injuries.
From the AP:
There were a number of [...] incidents [...] which came as about 5,000 Russia fans waving their country's flag marched to the stadium in a show of patriotism seen as provocative to many Poles.
The march was considered a huge security challenge and police were bracing for possibly more trouble after the match.
The two countries share a difficult history, including decades of control by Moscow over Poland during the Cold War. Many Poles felt the Polish authorities should not have allowed the Russians to march as a group in Warsaw given the historical wounds.
Inside the stadium things were decidedly calmer, although Russian fans did still get their point across by unfurling a giant "This is Russia" banner before kickoff.
Video of the various skirmishes on the streets this way...