In a row that echoed last year's controversy over a law banning women from wearing full veils on French territory, Muslim leaders denounced a
Communist-run town council's dismissal of the workers on health and safety grounds.
The four had been employed temporarily by the town of Genevilliers in the Paris suburbs to help run a summer holiday sports camp in southwestern France.
They were dismissed on July 20, the first day of Ramadan, after an inspector visited the camp and told them they were endangering children's safety by not eating or drinking between dawn and dusk.
Although they were fully paid for the week they had remaining on their short-term contracts, the instructors plan to contest their dismissal through labour courts.
The French Council of the Muslim Faith (CFCM) described the town's actions as "an attack on religious freedom" and said it was considering pressing charges against Genevilliers council for discrimination.
CFCM President Mohammed Moussaoui added: "Hundreds of millions of people fast for Ramadan every year without it having any impact on their professional activities."
Genevilliers Mayor Jacques Bourgoin defended the decision to remove the four employees from the camp, a stance which won strong backing from the far-right National Front.
"They did not respect the terms of their contract in a way that could have endangered the physical safety of the children they were responsible for," said a statement issued by the mayor's office.
"This lack of nourishment and hydration could have resulted in these employees not being in full possession of the means required to ensure that activities at the camp were correctly and safely run, as well as the physical safety of the children in their charge."
Nicole Varet, an aide to the mayor, said the decision to dismiss the four employees had been influenced by an incident three years ago in which a fasting camp worker had been taken ill while driving, resulting in an accident in which a child was seriously injured.
Genevilliers town hall later issued a statement saying that in order to avoid heightening tensions further, it would for the month of August drop the clause from contracts for workers at the camp that obliged them to eat lunch.
The four sacked workers believe the safety argument is a smokescreen for anti-Muslim prejudice.
One of them, who asked to be identified only by his first name, Samir, said their treatment had been "unfair and unacceptable" and that he was glad it had been brought into the public domain.
"We are thinking about going to court to get clear answers to our questions," he told AFP. "Do people have the right not to eat during the day?
Are doctors who observe Ramadan putting their patients' lives in danger?"
A spokesman for the National Front said the Gennevilliers mayor had made the right decision, adding that: "Those who oppose this wise decision are
making a mockery of the principles of safety and secularism."
The row over the Ramadan sackings erupted as France brushed off US State Department criticism of its ban on veils which fully cover women's faces,
introduced last year by the administration of former president Nicolas Sarkozy.
In its 2011 International Religious Freedom Report, the State Department expressed concern over a "rising number of European countries, including
Belgium and France, whose laws restricting dress adversely affected Muslims and others".