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Polish media have reported that the policeman sustained facial injuries when he tried to intervene in an area close to where US rock band Allah-Las, whose concert in Rotterdam was cancelled yesterday following a tip-off from Spanish police over a terror threat, are due to perform tonight.

The officer was treated at the scene and his injuries were not deemed to be life-threatening.

Police were alerted after the man was spotted walking in the Polish capital holding a knife and acting suspiciously.

The incident occured at around 3.30pm local time this afternoon and local media reports claim the perpetrator was a 24-year-old Israeli man, although this has not been confirmed by authorities.

Police spokesman Rafał Retmaniak said: "We do not know if this event was related to this concert or any other event in Warsaw.”