The boys, named as Damien, 12, and Hugo, 10, by authorities, had been forced to sleep on bare mattresses surrounded by their own filth for the past three years while their father, 39, step-mother, 44, and her seven-year-old son lived upstairs.
They were allowed out to attend the local state school in the day, but at night were locked into the cellar which had no natural light, electricity or toilet, and was never cleaned.
They were often beaten with a belt by their father, it is alleged, and forced to go without food.
The shocking conditions the children lived in became apparent last Wednesday night when the older boy, Damien, escaped and ran to the local police station in Bondy, Seine-Saint-Denis, to the north-east of Paris.
He told officers about the conditions he and his brother lived in, and officers went to their house in the suburb to see for themselves.
Policeman Deve Maboungou said: "When the police arrived they found a mattress and excrement - it was just as the boy had said."
Officers found the boys had 'scratches and burst blood vessels on their arms'.
A source told the Telegraph: "They slept on mattresses on the floor, without sheets, in the middle of their own mess in the cellar which was in an advanced state of decay."
The boys' mistreatment is said to have begun when their drug-addict mother moved out and their father moved in with another woman who had two sons, now aged seven and 17. The older boy is said to be a 'petit rat' student dancer at the Opera de Paris, which is looking after him.
The seven-year-old is said to have lived a very different life, attending private school and sleeping upstairs with the two adults.
The father and stepmother are now in custody as the case is investigated.