Russian and Ukrainian soccer fans scuffled briefly in the streets of Lviv in the early hours of Saturday after watching a Euro 2012 game in the city's fan zone, eyewitnesses said.
Around 15 Russian fans, delighted at their side's impressive 4-1 win over the Czech Republic, left the zone and started shouting and swearing at a smaller group of Ukrainians, said Reuters photographer Marian Striltsiv.
The two sides exchanged a few punches before local police quickly moved in and separated the combatants. No arrests were made.
One picture taken by a local agency showed a man with blood streaming from his nose.
Police denied there had been any violence and said they had prevented the two sides from fighting. Tensions between the two nationalities are particularly high in Lviv, in the far west of the country, in part because of Ukrainian resentment at Soviet rule from 1939 to 1991.
The Lviv region was the centre of a resistance campaign by Ukrainian nationalists fighting for independence from the Soviet Union during and after World War Two.
One of the main nationalist leaders was Stepan Bandera, a man still widely regarded as a hero in western Ukraine and as a bandit by Russia. Fans of local soccer side Karpaty Lviv always show a huge banner of Bandera during home games.
Photograph: Marian Striltsiv/Reuters