Counter-terrorism police in the UK are investigating a possible hate crime after several people received letters calling on them to attack Muslims on April 3, termed in the leaflet as "punish a Muslim" day.
Several Britons in London, the West Midlands and Yorkshire said they received the printed letter through the post, according to reports.
Some uploaded images of the communication on social media.
The leaflet says: "They have hurt you, they have made your loved ones suffer. They have caused you pain and heartache. What are you going to about it?"
It goes on to offer rewards for attackers, from 10 points for verbally abusing a Muslim and 50 points for throwing acid in the face of a Muslim, to 1,000 points for bombing a mosque and 2,500 points to "nuke Mecca".
"Public safety remains our priority and I would urge our communities to be vigilant but not frightened," West Yorkshire police said in a statement. "We are stronger when we stand together as one and will not be divided."
The Church of England's head of interfaith relations, Andrew Smith, tweeted: "Angry to hear about appalling 'punish a Muslim' letters that have been sent. Can't imagine how horrible & frightening it must be to get a letter like that. Time to redouble our efforts to build peace in our society & support those feeling scared or intimidated. The work goes on."