German and British police warned that the World Cup's off-the-field calm could be tested Tuesday as both Germany and England play key games.

About 50,000 England fans are expected in Cologne for their team's night match against Sweden. By that time, fans across Germany may be celebrating their team's earlier game against Ecuador in Berlin.

Even though both teams already are through to the next round, authorities have expressed concerns that hooligans from both countries could clash with each other and police.

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Early Tuesday morning, a dozen England fans were arrested in Cologne after tossing bottles at German riot police. Sixteen officers were injured, but none seriously.

And more fans are coming. At Cologne-Bonn Airport, more than 100 fan-filled charter flights from both England and Sweden were scheduled to land throughout Tuesday.

"I think it's a very difficult day," said Stephen Thomas, a British constable working with German and Swedish police in Cologne. "It's going to be the most challenging day of the tournament" to date.

"It is going to be a challenging day, but we are prepared," said Cologne's police superintendent Stefan Schwarz.

Police said the incident at Cologne's downtown square began after an Englishman scaled a statue to plant a flag on its outstretched arm, but lost his balance and fell several feet to the street. He was not seriously injured.

As German police moved to provide first aid, Thomas said, members of the surrounding crowd threw glass bottles. The resulting brief melee, during which police used pepper spray to control some in the crowd, resulted in the dozen arrests.

The fans were to be released later Tuesday and issued orders to leave the city.

"It was a very small disturbance by previous experiences," Thomas said.

It was also an exception at this World Cup. Since the tournament started June 9, hundreds of thousands of fans have watched games and partied; only an approximate 2,700 have been arrested or detained by police related to the World Cup across Germany, most for minor crimes, Thomas said.