"My daughter was raped post-mortem. They raped her to death, they drugged her to death and they continued to rape her," Montero said.
"With this sentence, Lucía has been murdered again. We cannot keep losing our daughters. We cannot allow this to continue happening," she said.
Women of all ages, carrying violet and green scarves that symbolise the feminist and legal abortion movements in Argentina joined the protest. Protesters also carried signs bearing the faces of ruling judges Facundo Gómez Urso, Aldo Carnevale and Pablo Viñas.
Pérez's body was found in October, 2016. Her death prompted nationwide protests calling for action against violence against women, as the cause was taken up by the Ni Una Menos movement against gender violence. The outcry resulted in the first ever women's strike being called.
Among groups and individuals who have expressed concern over the court's ruling was the Organisation of American States (OEA), which is calling on Argentina's Supreme Court to review the decision and "offer fair access to justice for Lucía's family and for all women victims and survivors of violence."
The court's ruling in the case said that the girl had consensual sex with one of the men and that she died of an overdose.
From Mar del Plata's Oral Criminal Court No. 1, Judges Urso, Carnevale and Viñas ruled there was a lack of evidence to find the accused men — Matías Farías, Juan Pablo Offidani, and Alejandro Maciel — guilty of sexual abuse or femicide, suggesting Perez's body did not show signs of rape.
The judges' description of the events that culminated in her death — and other statements, including that "Lucía had sex with whomever she wanted" — have prompted outrage.
"We must understand that just because a body does not show physical traces, does not mean there has not been gender-based violence", said Lauren Malacalza, coordinator of the Gender-based Violence Observatory of the Buenos Aires Province Ombudsman's Office.
"The murder of Lucía takes place in the context of three adult men using drugs to relate to her, a minor," she told the leftist CosechaRoja news portal.
The court found Farías, 25, and Offidani, 43, guilty of supplying drugs to a minor. They were sentenced to eight years in prison and a AR$135,000 fine.