Five days of opening World Cup calm ended with a volley of bottles and chairs as dozens of Germannd German police stepped up their border patrols.

Police detain a German soccer fan after the World Cup 2006 soccer match between Poland and Germany in Dortmund.[Reuters]

Police in Dortmund said they were prepared, trolling the crowds for known troublemakers. Among those arrested near the stadium were four Polish men found to have knives.

Under German law, people can be held up to 48 hours if police think they may commit a crime.

Most Polish fans returned home directly after the match to avoid high hotel costs in Germany. Around 7,000 attended the match, and thousands more made the trip to Dortmund to support their national team from outside.