L'Oreal heiress Liliane Bettencourt, 87, was expected to be asked to explain two undeclared Swiss bank accounts and an island in the Seychelles mentioned on secret recordings made by a former butler published in the media.
She has said she would repatriate the money from Switzerland and sort things out with the tax authorities.
She may also be quizzed about allegations by a former bookkeeper that Bettencourt and her late husband made big illegal cash donations to conservative politicians, including Labour Minister Eric Woerth for President Nicolas Sarkozy's 2007 election campaign.
Bettencourt was questioned "as a witness" at her villa in the wealthy Paris suburb of Neuilly, lawyer Georges Kiejman told Reuters.
Others involved in the case were summoned to the headquarters of the Paris financial crimes division, some of them held in custody.
Woerth has denied receiving any illegal money and rejected accusations of a conflict of interest over his wife's job with Bettencourt's wealth manager while he was budget minister in charge of tax affairs and treasurer of the ruling UMP party.
He is expected to be questioned by police later this week after the cabinet gave the green light last Wednesday. Woerth has refused to resign and says he will continue to lead a major pension reform due to be adopted by parliament in October.
What began as a family feud over lavish gifts by Bettencourt to a close friend, society photographer Francois-Marie Banier, has turned into a political scandal.
Francoise Meyers-Bettencourt, the billionairess's daughter, has filed two lawsuits seeking to have her mother declared mentally unfit and made a ward of the court.
Public prosecutor Philippe Courroye wrote to Meyers-Bettencourt last week saying her application had no chance of acceptance without a medical certificate attesting to her mother's mental state.
The heiress has refused to undergo an independent medical examination but said in the statement on Friday she was fully capable of looking after her remaining fortune.
She said she had given her daughter and grandchildren her 30.98 percent stake in L'Oreal, with a market value of 15.6 billion euros ($20.09 billion), but receives the dividend income and has about 1 billion euros in other assets.
The secret recordings suggest Bettencourt's wealth manager received advice from President Nicolas Sarkozy's legal counsellor at the Elysee presidential palace on how to deal with the lawsuits.
