Poland's security services have increased their readiness level after a border patrol found explosives a cellphone on a raft crossing the Bug river on the border of Poland and Ukraine. The phone had a photo of Warsaw's National Stadium in its memory.

"We have received certain information that pointed to a potential appearance of a terrorist threat," said Interior Minister Jacek Cichocki. "But there is no indication of a concrete danger for a specific place or person."

Cichocki said all Polish security services, not just those involved in Euro 2012 operations, would be on higher alert.

There have been no major security incidents during the three-week tournament, but Polish police were criticized for not intervening during street fights between local hooligans and Russian fans before their teams played June 12.