The cemetery contains the graves of tens of thousands of Muslim soldiers
Police have arrested four people in connection with the desecration of nearly 150 Muslim graves at a French World War I cemetery at the weekend.
President Nicolas Sarkozy has condemned the desecration and ordered an inquiry.
A pig's head was hung on one headstone in the cemetery, which holds the graves of tens of thousands of Muslims, mainly from French colonies in Africa.
A year ago, a group of neo-Nazis drew swastikas on 50 Muslim graves at the cemetery, France's largest from WWI.
In the latest incident, slogans insulting Islam and France's justice minister, who is of North African descent, were written on some of the graves.
About 78,000 colonial subjects of France, including many Muslims from North Africa, died in the war.
The Notre Dame de Lorette war cemetery, near Arras in northern France, is on the site of some of World War I's largest battles.
Two men were sentenced to a year in prison for the desecration of about 50 graves in the Muslim section of the cemetery in April 2007.