The letter is being circulated by lawyers at the Christian Legal Centre who have been supporting Oluwole Ilesanmi since his brief detention last weekend.
Addressed to Home Secretary Sajid Javid, the note says: "It's not the first time a Christian street preacher has been wrongly arrested in the UK.
"Christian street preachers should be free to share the gospel, even where it means challenging the beliefs of others."
The campaign demands an urgent review to ensure police are properly trained to "act within the law" in their dealings with street preachers.
The CLC, which is part of the Christian Concern campaign group, insists evangelists should be free to share the Gospel.
Are you appalled by the treatment of this street preacher in London? We have launched a global petition calling on the government to do more to protect Christian #streetpreachers in the UK. Please sign and share. @AntisemitismEyehttps://t.co/inCQoAYYKg#FreedomOfSpeech pic.twitter.com/nXBbFxrnjr
— Christian Concern (@CConcern) March 1, 2019
The petition continues: "Too often, police officers have shown themselves either to be ignorant of this freedom, or unwilling to uphold it.
"This leads to a chilling effect, where people are increasingly unwilling to say what they believe, for fear of arrest.
"Please urgently investigate what guidance and training is being given to police officers nationwide to ensure that they protect the freedom to preach in public."
The CLC says it has previously cleared the names of scores of street evangelists who have been arrested and, in some cases, charged.
Dear @obianuju thank you so much for your coverage on this case. We thought you would like to see this update and thank you for support from #streetpreacher Olu himself. #London #Southgate #FreedomOfSpeech pic.twitter.com/24wII42pbs
— Christian Concern (@CConcern) February 28, 2019
Last week, Premier reported how the Metropolitan Police was conducting an review of Mr Ilesanmi's arrest.
The force said it came after complaints were received and the 64-year-old's detention was made to "prevent a breach of the peace".
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