Radical sermons are openly delivered in Britain. However, a Jamaican-born convert to Islam was sentenced to nine years in prison in March last year for urging followers to kill Hindus, Jews and Americans.
In Germany, authorities are aware of mosques where radical sermons are delivered, but avoid crackdowns.
The rector of the Paris Mosque, Dalil Boubakeur, known for his moderate stance, approved of the detention of the Turkish imam.
"We have to work hand in hand ... so that the function of the imam, the status of the imam in France, is void of any suspicion," Boubakeur said on Monday.
It is the status of the imam that is the crux of the problem.
With its estimated 5 million Muslims -- the largest Muslim population in western Europe -- France wants an "Islam of France" to emerge that reflects French values.
Far from that, the majority of imams in France, estimated at 1,500, come from abroad and less than half speak French.
About 30 mosques in France are controlled by the hard-line Salafist movement that preaches a strict interpretation of the Quran, the Muslim holy book, the newspaper Le Monde reported, quoting intelligence sources.
The isolation of Muslim youths in working class suburbs, France's version of ghettos, has made them particularly vulnerable to teachings of radical imams, said Berthomet, the former anti-terrorism officer.
"These radicals penetrate some suburbs more easily than social workers or police," he said.
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