West Yorkshire police said they were taking the threats "extremely seriously” and had increased patrols in Hanover Square, a mainly Muslim inner-city area where at least two residents received the letters last week.
The literature shows an image of a sword and the St George’s flag with the words: "Kill scum Muslims”. It questions why Muslim women wear burqas then states: "We are now going to do acid attacks on anyone who wears the funny black masks around your square & Bradford & other places.”
It alleges that "three of ur male pigs” were grooming four white girls, then states: "We know who the three male pigs are they are walking dead pigs.”
Police carried out extra patrols over the bank holiday weekend on Hanover Square, in the Manningham area of Bradford near the city centre. Naz Shah, the Bradford West MP, said she had reported the letters to police on Saturday.
Mohammed Qayd, 27, one of the recipients of the letter, said his mother was afraid that she would be targeted and that the threats had come "completely out of the blue”.
"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.”
Qayd said the letter was delivered by second-class post and had a Lancashire postmark, but that he believed it had been sent by someone who knows Hanover Square and its residents.
Another recipient, who did not want to be identified beyond his first name, Shoab, said the letter could raise tensions before a planned English Defence League (EDL) march in Bradford on Saturday, which coincides with Eid celebrations.
He said: "The police mentioned the EDL will be marching. It can worry you if you really think about it – what if they’re something to do with it? But the EDL have come and gone before, sometimes something tends to happen. It could erupt, you see.
"Eid could be Friday or Saturday – a lot of times we’ll all be out to families, people go out to restaurants in the town centre. There’s always that chance something could happen.”
Outside the Masjid Nimrah mosque, which broadcast a call for prayer on Monday lunchtime, Masood Ahmad, 43, said the threats had concerned residents. "Everybody’s shocked. There’s a lot of women in the street wearing the burqa,” he said. "I didn’t know about the EDL [march] but if this spreads out it might create a problem. It might be chaos here.”
The letters follow a sharp rise in Islamophobic hate crime after the terror attacks on Westminster, Manchester and London Bridge. The spike in anti-Muslim incidents is thought to have fallen to normal levels following a surge of 500% in Greater Manchester in the month after the arena bombing.
A West Yorkshire police spokeswoman said: "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.”
Nadeem Murtuja, the chair of advocacy group Just Yorkshire, which was set up to restore race relations after the Bradford riots in 2001, said she was extremely concerned that Muslims were having to shave their beards or remove hijabs as a result of Islamaphobic threats.
"This is a time for calm heads and people in positions of power, locally and nationally, need to become more visible in their stance towards protecting the human rights of all communities, including the Muslim community.”