An investigation has been launched after Muslim families were sent letters warning they would be killed and attacked with acid in Bradford.
West Yorkshire Police said they are investigated the letters to addresses in Hanover Square "extremely seriously" and are treating them as suspected hate crimes.
One of the letters, seen by Tell Mama, who records anti-Muslim attacks, contains a threat to "kill scum Muslims" over the top of an image of the St George's flag.
It continues: "Why do Muslim woman [sic] wear burkas? Because they are f*****g ugly and would frighten the dogs in the street."
Whoever wrote the letters adds how they will "do acid attacks" on "anyone who wears the funny black masks around your square and Bradford and other places". The letter also alleges "there are three of ur [sic] male pigs who are grooming four white girls".
The threat comes after a string of acid attacks which have occurred across the UK over the last few months. The government announced punitive measures of life sentences for those who commit such attacks as part of plans to tackle the fast-growing crime.
Mohammed Qayd, 27, one of the recipients of the letter in Hanover Square, said it arrived "comepletely out of the blue".
He told the Guardian: "I was shocked. My mum wears a burqa and she goes to town regularly so I was concerned for her. When I explained it to her she realised the severity of it and was afraid. You start wondering whether it's safe to go out on their own."
The letter, sent via second class post with a Lancashire postmark, arrived as the English Defence League (EDL) are scheduled to stage a rally in the Bradford borough of Keighley against the "outrageous sexual grooming and abuse of English girls by Muslim men" in the Bradford borough of Keighley on 2 September.
West Yorkshire Police has now confirmed they have launched an investigation into the letters sent to Bradford.
A spokesperson said: "The incident is being treated as a hate crime and being taken extremely seriously.
"A thorough investigation has been launched and officers are working with the local community and partners to identify and prosecute those responsible for this despicable crime.
"We understand the impact hate crime and hate incidents can have on our communities and on individuals, and crimes of this nature will not be tolerated.
"Additional patrols have been deployed to the area following this report and police are working with local leaders to reassure residents, and to trace those responsible."