Polish police have arrested a string of young people, mainly for making threats related to school attacks, they said on Friday, as authorities sought to calm fears spurred by rumours online of a "Bloody Friday" after two stabbing sprees in as many days. Social media in Poland has been filled with talk of a supposed online challenge that could be behind the attacks, but police on Friday said their investigations had found no evidence of this. The incidents have prompted a nationwide security response, with police deployed to 10,000 schools and the government warning against the spread of misinformation that officials fear could inspire copycat attacks. "There is a lot of emotionally charged misinformation circulating online – spontaneous reactions born of understandable anxiety – but there is no sign of any nationwide 'challenge' taking place," Digital Affairs Minister Krzysztof Gawkowski said.