Under the headline "That's It, They're Coming", is an image of a crowd of migrants, all of them men, outside the cathedral of the southern town of Beziers.
"The state is imposing them on us," the poster reads. "Migrants in our town centre."
The poster put out by Mayor Robert Menard, an ally of the far-right National Front, is a reaction to government plans to relocate thousands of migrants from the north coast to dozens of sites around France.
Emmaus, a French group that works with migrants and other homeless people, denounced Menard's poster as "nauseating" in a post on its Twitter account.
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SOS Racism has called for the region's governor to step in and stop Menard's planned referendum.
And the government's anti-racism body DILCRA has called on the prosecutor's office in Beziers to investigate what it says is a "flagrant" example of incitement to hatred.
Speaking to French radio today, Menard said: "I'll do everything to ensure that these migrants don't settle in."
Beziers' mayor since 2014, Menard said he had not been informed that a migrant reception centre was planned in the town and thought residents should be allowed to vote in a referendum asking whether the migrants should be accepted.
The government plans to break up the "Jungle", a shantytown near Calais in northern France currently occupied by up to 9,000 migrants.
A few towns have seen anti-migrant demonstrations and some of the reception centres being prepared have been attacked.
Menard was for 23 years the head of the media rights group Reporters Without Borders, which has distanced itself from him since he left in 2008.