It came as racial tensions grow in the country as it prepares to introduce a total ban on burkas and other forms of religious dress which cover the face.
At one point the lawyer, who was out with her daughter, is said to have likened the Muslim woman to Belphegor, a horror demon character well known to French TV viewers. Belphegor is said to haunt the Louvre museum in Paris and frequently covers up his hideous features using a mask.
An argument started before the older woman is said to have ripped the other woman's veil off. As they came to blows, the lawyer's daughter joined in.
"The shop manager and the husband of the Muslim woman moved to break up the fighting," the officer said. All three were arrested and taken to the local gendarmerie for questioning.
A spokesman for Trignac police said that two complaints had been received, with the Muslim woman accusing the lawyer of racial and religious assault. The latter, in turn, had accused her opponent of common assault.
The French parliament has adopted a formal motion declaring burkas and other forms of Islamic dress to be "an affront to the nation's values." Some have accused criminals, from terrorists to shoplifters, of wearing veils to disguise themselves.
A ban, which could be introduced as early as the autumn, would make France the second country after Belgium to outlaw the Islamic veil in public places.
But many have criticised the anti-burka lobby, which includes the French President Nicolas Sarkozy, for stigmatising Muslim housewives.
Many French woman from council estates are forced to wear the veils because of pressure from authoritarian husbands.
The promise of a ban has prompted warnings of racial tensions in a country which is home to some five million Muslims – one of the religion's largest communities in Europe.
Mr Sarkozy's cabinet is to examine a draft bill which will impose one-year prison sentences and fines of up to £14,000 on men who force their wives to wear a burka.
Women themselves will face a smaller fine of just over £100 because they are "often victims with no choice in the matter", says the draft.
The law would create a new offence of "incitement to cover the face for reasons of gender".
And it would state: "No one may wear in public places clothes that are aimed at hiding the face."