“Potentially malicious communications” have been sent to addresses across the country encouraging people to take part in what the writers call “Punish a Muslim Day”.
The letters have been received in Bradford, Leicester, London, Cardiff and Sheffield.
It is believed they originated from the Sheffield area and allegedly share a similarity to letters sent to mosques in the UK and US in May 2017, which were also posted from Sheffield.
A spokesperson from West Yorkshire Police appealed to the community that “everything possible is being done to ensure their safety” and “we are stronger when we stand together”.

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