"Punish A Muslim Day" letters are being sent to families in East London.
One letter was sent to the business address of Liberal Democrat councillor Riaz Ahmed in Bradford.
'It seems odd that anyone would send something like this to an address in a predominantly Muslim area.
Home Office minister Victoria Atkins said in Parliament: "The Government condemns the content of these letters as clearly abhorrent with no place in a decent society".
The letter, which bears an image similar to the royal coat of arms, says: 'Sheep follow orders and are easily led, they are allowing the white majority nations of Europe and North America to become over-run by those who like nothing more than to do us harm and turn our democracies into Shaira led police states'.
There is also an image of a dagger with the letters MS, suggesting it is linked to so-called Muslim Slayer who targeted mosques in London and the United States a year ago with similar threats.
For example, removing a woman's hijab is worth 25 points, murdering a Muslim is valued at 500 points, and bombing a mosque 1,000 points.
"Officers are now investigating the full circumstances", the statement reads, after Londoners hailing from Bradford, Leicester, London and Cardiff and Sheffield complained that they received the letter.
LETTERS encouraging United Kingdom residents to take part in "Punish a Muslim Day" have reportedly been sent to addresses across the country.
Iman Atta, director of anti-Muslim hate monitoring service Tell MAMA, said: "This has caused quite a lot of fear within the community".
Collaborating with the Metropolitan Police, the organization based in London urged the ones who received these letters to report it or ring a non-emergency number, 101.
"(The matter) is being treated with the utmost seriousness", the organization said.
'It is essential that all letters and envelopes are kept and handled minimally to preserve evidence for the police to investigate.
Police forces across the country are continuing to investigate and are were working with anti-racism groups like Tell Mama UK.
Tory MP Anna Soubry said: "This isn't really a hate crime, this is an act of blatant incitement to terrorism".
According to the NGO, "British Muslims have been targeted (...) A national operation- co-ordinated by Counter Terrorism Policing North East - has been launched after a malicious letter containing messages of hate crime was reported to police by a number of people on Friday (9 March) and over the weekend".
"These communications are extremely distressing and we appreciate that members of our communities will be very concerned", Mr Forber said.
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