Angry protesters have clashed with police and smashed windows after an Argentine court acquitted 13 people charged with running a sex slavery ring.
Demonstrators furious over the legal ruling took to the streets of Buenos Aires and in at least seven provinces on Wednesday.
Susana Trimarco, an activist who personally freed many sex slaves as she searched for her missing daughter Marita Veron, spoke with President Cristina Kirchner, who was among those angered by the verdict.
"I had a call from the president and she was shouting 'I cannot believe it. I cannot believe it'," she said.
"Even the wife of [US] President [Barack] Obama voiced her support to me. I thank her from the bottom of my heart and assured her that we are not going to stop fighting," she added, referring to the US first lady, Michelle Obama.
In Buenos Aires, demonstrators gathered outside the local office for Tucuman province, the state where the ruling was passed. They threw rocks and other heavy objects at windows, smashing them.
In Tucuman itself, where Veron went missing a decade ago, a large procession of people marched with a banner that read, "Justice for Marita".
Disappointment at the verdict
In 2008, Trimarco's campaign won a toughening of sentences for those convicted of sex trafficking, but she has yet to find her daughter, who vanished in 2002 at the age of 23.
Trimarco and fellow campaigners believe that Marita is among the victims of an alleged sex ring in Tucuman province in northern Argentina.
"The court system does not understand the system of people trafficking," lamented Zaida Gatti, who leads the state-run National Programme to Rescue People Harmed by Trafficking Crimes.
Jose D'Antona, Trimarco's attorney, said he planned to seek the impeachment of the three judges who delivered the verdict, a call that was endorsed by several political leaders.
A disappointed Kirchner, a lawyer herself by training, said she hoped more "democratisation" could be brought to Argentina's court system.
Other party leaders from across the political spectrum expressed similar disappointment with the verdict.
Prosecutors in Tucuman had sought between 12 to 25 years in jail for those accused in the case. The grounds for the court ruling were not immediately made public.
Prostitution remains legal in Argentina, but managing brothels and trafficking in people have been federal crimes since 2008, under a law Congress passed after lobbying by Trimarco.
Trimarco's search for Veron exposed an underworld of organised crime figures who operate brothels with protection from authorities across Argentina.
The security ministry says that since the passing of the new law in 2008, 938 people were rescued from trafficking rings - 215 of them from the sex trade, and 723 from other workplace exploitation.
Hundreds of women also have been saved by a foundation Trimarco created in her daughter's name in 2008.
The foundation also provides legal help, but its lawyers have found that proving sex slavery is difficult without full support
from the same police who often get paid to protect prostitution rings.