The pontiff said religions had to be treated with respect, so that people’s faith was not insulted or ridiculed.
To illustrate his point, Pope Francis told journalists that his assistant could expect a punch if he cursed his mother.
The remarks came as funerals were held for four people killed in the Charlie Hebdo attack by militant Islamists.
Friends and family paid last respects to cartoonists Bernard Verlhac, known as Tignous, and Georges Wolinski, as well as columnist Elsa Cayat and policeman Franck Brinsolaro.
Eight magazine staff, a visitor to the magazine, a caretaker and two policemen died in the last week’s attack. A policewoman and four people at a kosher supermarket died in separate attacks.
Al-Qaeda said it had directed the Charlie Hebdo attack.
Charlie Hebdo was targeted for publishing cartoons of the Prophet Muhammad. It printed another cartoon of the Prophet on its front page after the attacks, angering some Muslims who say all depictions of the Prophet should be forbidden.
France has deployed thousands of troops and police to boost security in the wake of last week’s attacks. There have been retaliatory attacks against Muslim sites around France.
Speaking to journalists flying with him to the Philippines, Pope Francis said last week’s attacks were an “aberration”, and such horrific violence in God’s name could not be justified.
He staunchly defended freedom of expression, but then he said there were limits, especially when people mocked religion.
“If my good friend Doctor Gasparri [who organizes the Pope’s trips] speaks badly of my mother, he can expect to get punched,” he said, throwing a pretend punch at the doctor, who was standing beside him.
“You cannot provoke. You cannot insult the faith of others. You cannot make fun of the faith of others. There is a limit.”
In a separate development, the government announced that a Malian employee of the Jewish supermarket that was attacked would be given French citizenship.
Some 300,000 people signed an online petition calling for the move after the Muslim employee, Lassana Bathily, hid several customers from gunman Amedy Coulibaly in a cold store.
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