French police said it was holding a man on Friday for threatening a school principal who turned away his daughter for wearing a traditional Muslim abaya dress.

President Emmanuel Macron's government announced last month it was banning the abaya -- a garment worn by Muslim women that covers the body from the neck to the feet -- in schools, as it broke the rules on secularism in education.

The man, arrested Thursday, was being prosecuted for "issuing threats designed to intimidate a person charged with an official mission", the local prosecutor Dominique Puechmaille told AFP.

The man's daughter was stopped at the entrance of her high school on Thursday and asked to remove her abaya. When she refused, she was stopped from entering, police said.

Her father then telephoned the school and spoke first to a guard and then to an educational advisor. He is accused of having issued death threats targeting the school principal in both conversations.

French Education Minister Gabriel Attal called the threats "intolerable and unspeakable".

The principal was now under police protection, he said.

The president of the Auvergne-Rhone-Alpes region, Laurent Wauquiez, added that the school staff had received threats "of death and decapitation".

The authorities had to show "firmness and determination" in the face of such threats.

- Court rejected challenge -

The ban was quickly challenged before France's State Council, the highest administrative court, by an association arguing the ban could incite hatred against Muslims and racial profiling.

On Thursday however, the court threw out the complaint.

Wearing the abaya "follows the logic of religious affirmation", said the ruling, adding that the decision was based on French law, which does not allow anyone wearing visible signs of any religious affiliation in schools.

The government ban did not, it said, cause "serious or obviously illegal harm to the respect for personal lives, freedom of religion, the right to education, the well-being of children or the principle of non-discrimination".

On the first day of the school year on Monday, French schools sent dozens of girls home for refusing to remove their abayas -- a shoulder-to-toe over-garment.

Nearly 300 schoolgirls defied the ban on that day, Attal said. Most agreed to change garments but 67 refused and were sent home, he said.

Around 10 percent of France's 67 million inhabitants are Muslim, according to official estimates.

Most have origins in the northern African countries Algeria, Morocco and Tunisia, which were French colonies until the second half of the 20th century.