Tusk issued the warning at a national rally of the Polish Teachers' Union on Friday.
"The war in the east is entering a decisive stage," Tusk said. "We can feel the unknown is coming.
"No one among us knows the end of this conflict, but we know that it is now growing to dramatic proportions," he continued. "The past dozen or so hours show that the threat of a global conflict is serious and real."
Stressing the importance of teachers' work in ensuring Poland security going forward, Tusk also said that "the safe, strong and resilient nations are those which enjoy technological, intellectual and creative advantages." (PAP)
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