A 15-year-old is facing possible prosecution for holding up a placard which branded Scientology a "cult".
The teenager held up a sign which read, "Scientology is not a religion, it is a dangerous cult" in May, outside its headquarters in the City of London.
City of London Police said they had received complaints and warned the teenager to get rid of the sign as it breached the Public Order Act.
A file is being passed to the Crown Prosecution Service.
Writing on an anti-Scientology website, the teenager says: "I need precedents, legal advice, definitions and defences.
"I intend to make a big folder with all the defence you can give me, and in case this does get through to court, I will be well prepared."
Ian Haworth from the Cult Information Centre said he would avoid labelling any organisation a "cult", but said bringing the issue into criminal rather than civil law would be "very serious".
He added: "If it wasn't so serious it would be farcical.
"I'm very upset by what appears to have happened. I hope the CPS realise that this is an error and that nothing happens to this young man."
A spokeswoman for City of London Police said they had received complaints about the use of the words "cult" and "scientology kills" and warned protestors their signs breached the Public Order Act.
Chief Supt Rob Bastable said: "City of London Police upholds the right to demonstrate lawfully, but we have to balance that with the right of all sections of community not to be alarmed, harassed or distressed as a result of other people's behaviour."