The inscription Muhammad lies here :naooz: was among the Islamophobia-fueling slogans found at the French-speaking Charleroi city, AFP reported.
According to a statement released by Xavier Godefroid from the mayor's office on Tuesday, the opponents of the construction of the mosque have left many unacceptable, intolerable and disrespectful messages at the site.
Godefroid added that the city's police have begun an investigation into the case.
The rise in anti-Islamic and racist behavior in European nations such as France has attracted worldwide attention.
In a debate about the role of Islam in the French society, Interior Minister Claude Gueant referred to the number of Muslims in France as "a problem."
Paris had earlier declared that any woman -- French or foreigner -- who wears a niqab or burqa (an Islamic headdress) in public will be fined 150 euros, and those that force women to wear such covers will face a much larger fine and a prison sentence of up to two years.