Counter-terror police are investigating reports that people across the country have received letters encouraging them to participate in “Punish A Muslim Day”.
The A4 printed notes - which have been shared widely on social media - call on recipients to carry out “horrific” acts of violence against Muslims on April 3, saying a points system will be in place for attacks.
Counter Terrorism Policing North East, who are coordinating the investigation, said officers had received “a number of reports of potentially malicious communications”.
Meanwhile Tell MAMA, which monitors anti-Muslim activity in the UK, said it had been made aware of letters arriving in London, Yorkshire and the Midlands.
Campaigners and MPs have slammed the letters as “disturbing hate mail”.
The charity Muslim Engagement and Development (MEND) said it was “deeply appalled and horrified” by the letters, saying the notes must be treated with “every bit of seriousness”.
“This letter appears to be symptomatic of the rise in Islamophobia we have witnessed in recent times,” the organisation said in a statement.
“We call upon the government to take Islamophobia as seriously as other forms of hate crime,” it added, calling on anyone who has received one of the letters to contact the police.
Meanwhile, Bradford West MP Naz Shah said the letters had also been delivered in her city.
“People in Bradford and in other parts of the country have received very disturbing hate mail titled ‘Punish a Muslim’,” the Labour MP wrote on Facebook.
“Understandably, this is very distressful not only those who have received the letter but also for the wider communities.
“I would appeal to the wider community to remain vigilant and report any suspicious activity to the police.”
A police spokesperson said: “Police have received a number of reports of potentially malicious communications sent to individuals across the UK.
“Officers are currently investigating the full circumstances,” they continued.
“Counter Terrorism Policing North East are coordinating the investigation at this time and will consider any potential links to existing enquiries.
“Anyone with any concerns about a communication they may have received should contact their local police force.”