Police are investigating a possible hate crime after reports that anti-Islamic letters were posted across the country.
West Yorkshire Police confirmed it had around six reports of letters advertising "Punish a Muslim Day".
The force said it had received a couple of letters for further analysis to determine the full circumstances and their possible origin.
A police spokesman said: "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."
Social media users in London and Birmingham also reported receiving the letters.
A Metropolitan Police spokesman said it was not yet clear whether any criminal allegations relating to the letters had been reported in the capital.
Iman Atta, director of anti-Muslim hate monitoring service Tell MAMA, said: "This has caused quite a lot of fear within the community.
"They are asking if they are safe, if their children are safe to play outdoors. We have told them to keep calm, and to phone the police if they receive one of these letters."