Morocco’s penal code allows a rapist to marry his victim if she is a minor as a way of avoiding prosecution.
A 16-year-old girl, Amina Filali, killed herself a week ago after being severely beaten during a forced marriage to her rapist.
The protesters held signs saying, “The law has killed Amina”.
The parents of Amina Filali were at the protest, says the BBC’s Nora Fakim, in the Moroccan capital, Rabat.
They say their daughter was pressured by a local court into marrying her rapist, who then abused her.
She died after swallowing rat poison on 10 March.
Campaigners are also calling for the judge who allowed the marriage and the rapist to be jailed.