Vandals splashed blood on a wall and left a pig's head at a mosque under construction in the eastern French city of Belfort, a security official said on Monday.
Islam forbids its followers from eating pork. On Monday marks the festival of Eid al-Fitr, the end of the holy month of Ramazan, when Muslims fast from dawn to dusk.
France has Europe's largest Muslim community, numbering about five million, and mosques are occasionally vandalised. Swastikas and racist slogans were scrawled on the walls of two mosques and another was set on fire last month on the day French Muslims started celebrating Ramazan.
Eric Krust, deputy chief of public security in the Territoire de Belfort department said the pig's head was left on a windowsill along with splashes of blood on an area 5 metres (yards) long and 2.5 metres high
The plan to build the mosque has been criticised by local opposition politicians because of the site chosen in Belfort's old town.