They are being held at the headquarters of France's vice squad in central Paris, where they will remain for up to 48 hours while officers decide whether to refer their case to a judge.
One of the young women at the centre of the scandal testified earlier this year that she had had sex with the two players while she was still under the age of 18.
If they are charged and convicted of soliciting and paying for sex with a minor, the pair face three years in jail and a fine of $58,000 but only if prosecutors prove they knew she was underage.
Age 'not known'
Ribery, who plays for Bayern Munich and France, refused to answer questions from reporters as he arrived at the police station.
Ribery's lawyer, Sophie Bottai, told reporters in front of the police station that her client had had no idea the young woman was under 18 when he had paid for sex with her in 2009.
Real Madrid striker Benzema, 22, missed out on this year's French team despite playing in much of the World Cup qualifying campaign.
The young woman in question, Zahia Dehar, is now eighteen. She said that she sold sexual encounters to Ribery in 2009 and Benzema in 2008, when she was still underage. She said the players did not know her age.
In France the general age of consent is fifteen years old, but in the case of prostitution a young person remains legally a minor until he or she reaches 18.
The footballers became caught up in the investigation after detectives probing allegations of a vice ring bugged the Zaman Cafe, an expensive nightspot just off Paris's most glamorous street, the Champs Elysees.
Four people - including the owner of the club, a waiter who worked there and a man suspected of pimping call girls to celebrities and sportsmen - have been placed under formal judicial investigation and could face trial.