French President Francois Hollande says he has been particularly "hurt" by his former partner's claims that he secretly despised the poor.
Valerie Trierweiler, in her scathing memoir, claimed that the Socialist leader jokingly referred to the destitute as "toothless" in private.
She was the president's partner for nine years until he brusquely ended their relationship following revelations he had an affair with an actress.
In the book, entitled "Thank You for the Moment," Trierweiler writes that Hollande "portrayed himself as the man who doesn't like the rich. In reality, the president doesn't like the poor".
In an interview with Le Nouvel Observateur, published on Wednesday, the Socialist president defended his commitment to the needy as he struggles to stop his popularity ratings from sinking to new depths.
"I don't want people to be able to say or write that I don't care about suffering in society, because that's a lie that hurts me," Hollande said.
"This attack on the poor, the most disadvantaged, I took it as an attack on my entire life."
'Most unpopular president'
The president said in every political post he had held "I have thought only of helping and representing those who are suffering".
"I have never been on the side of the powerful, even if I am not their enemy, but I know where I come from," he said.
Trierweiler's memoir has already become a bestseller in France since it was published last week.
Its publisher said 145,000 copies were sold in the first four days and it has ordered a further print run of 270,000 copies.
However, two opinion polls showed that most French people disapprove of Trierweiler's book.
Fifty-six percent believe she was "wrong" to write the book, according to a poll by Harris Interactive for VSD magazine, while 67 percent were opposed to the memoir in a CSA survey.