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An office worker works at a desk suspended from the outside of a building at Peter The Pleater on February 29, 2012 in London, England. A Christian group has tried to place ads on 20 London buses encouraging to get help to "cure" their homosexuality. - 

David Brancaccio: In Britain's capital today, the mayor blocked an anti-gay group from putting ads on London buses.

Joining us live from London, is Marketplace's Stephen Beard. Hi, Stephen.

Stephen Beard: Hello David.

Brancaccio: So what are these ads?

Beard: A Christian group called Core Issues Trust paid about $15,000 to place the ads on around 20 London buses. These ads essentially promoted idea that gay people can be helped to give up their homosexuality and should do so. The ads were in fact a response to an earlier advertising campaign by a gay rights group supporting the British government's plans for gay marriage.

Brancaccio: So how did the mayor get involved in this?

Beard: Well, news of the ad campaign got out; it triggered a Twitter storm. And the mayor -- who's facing an election campaign at the moment -- stepped in and ordered his transport chief to pull the ads. He said London is one of the most tolerant cities in the world, and intolerant of intolerance.

However, Mike Davidson, the man who placed the ads, says he's been denied freedom of speech.

Mike Davidson: I think its very concerning that this level of censorship still exists particularly in a city that claims to be so tolerant.

He says the mayor is seeking favor with gay voters in London. And the group Index on Censorship, which campaigns for freedom of speech, has criticized the mayor's ban. It says it's part of a trend to shut down controversial issues rather than have a debate about them.

Brancaccio: Marketplace's Stephen Beard in London. Thank you very much.

Beard: OK David.