In France a male suspect, already under arrest in another case in the central town of Bourges, was being investigated, along with a young woman suspected of being a madam keeping an eye on the prostitutes on behalf of the pimps.
In Romania police acting alongside French investigators detained four suspected pimps expected to be transferred to French custody under a European arrest warrant.
Among them was a woman who police suspect to be the ringleader. Members of her family also appeared to be involved.
Police said she ruled over prostitutes working along a portion of a national highway in France's Loiret region where the women were forced to hand over the money they made.
Last week police in the southern city of Nice cracked a prostitution ring run from Romania, and in November another was broken up in northern France.
More than 80 percent of the up to 20,000 prostitutes in France are of foreign origin, against 20 percent in the 1990s according to a report to parliament.