Croatia became the first country to be charged over racism at Euro 2012 on Saturday and police and calculators were out in force before the final round of Group A matches got under way.
With Russia facing Greece in Warsaw and co-hosts Poland meeting the Czech Republic in Wroclaw to decide who qualifies for the quarter-finals, police pledged to protect thousands of fans expected in the Polish capital and avoid a repeat of Tuesday's violence.
Twenty thousand Russians and 4,000 Greeks have tickets to their match and Warsaw's 100,000-capacity fanzone is expected to be full.
Those numbers are a headache for police after Tuesday's problems when Polish hooligans set upon a group of Russian fans marching to the stadium for the politically-charged match between their respective teams.
"Smiling fans are welcome in Warsaw. But if we see others around like last time then we will detain them immediately," said Warsaw police spokesman Polish fan Maciej Kowalski.
Russia lead the group with four points with the Czechs on three, Poland on two and Greece on one.
If teams are level on points, their respective positions will be decided on their head-to-head record rather than old- fashioned goal difference, a system which is increasingly unpopular because of its complexity.
UEFA's disciplinary panel, which has been working overtime on a string of cases involving fireworks and missile-throwing by fans, opened proceedings against Croatia's Football Federation for racist chants by their fans against Italy.
The Football Against Racism in Europe (FARE) network, which works closely with UEFA and has two "international monitors" at each Euro 2012 game, tweeted on Friday that its observers reported "between 300 and 500 Croatian fans were involved in racially abusing Italy striker Mario Balotelli."
European soccer's governing body is still investigating reports of alleged racist chanting during Italy's match against Spain in Gdansk, and Russia's match with the Czech Republic in Wroclaw.
The issue of racism dominated the build-up to the tournament, co-hosted by Poland and Ukraine and the biggest sporting event in eastern Europe since the end of communism.
Croatia was fined 25,000 euros after fans threw flares and missiles on to the pitch against Ireland in Poznan, while smaller fines have been dished out to Germany and Portugal for similar offences.
Russia was fined 120,000 euros over the setting off of fireworks and display of "illicit banners" against the Czech Republic and warned it will be docked six points in the Euro 2016 qualifiers if there is a repeat.
With tensions mounting, UEFA president Michel Platini appealed to supporters.
"I appeal therefore to all fans that are going to Warsaw or Wroclaw tonight .....to conduct themselves with dignity and respect, and to behave themselves this evening at the stadiums and in the cities," he said in a statement.
(Writing by Brian Homewood, Editing by Ed Osmond)