An activist hurls stones at anti-riot policemen guarding the Tucuman provincial house in Buenos Aires.

Buenos Aires -

Angry Argentines clashed with police and smashed windows and politicians voiced outrage on Wednesday after a court acquitted 13 people charged with running a sex slavery ring.

Demonstrators furious over the legal ruling took to the streets of the capital and in at least seven provinces, including Tucuman in the north, where a court announced the acquittal on Tuesday.

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. Veron is believed to be among the victims of the sex ring.

“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 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 representing Tucuman province threw rocks and other heavy objects at the 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.”

In 2008, Trimarco's tireless 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. But on Tuesday a court in Tucuman found the suspects not guilty.

The verdict triggered national outrage, and protest groups called for marches in cities around the country.

“The court system does not understand the system of people trafficking,” lamented Zaida Gatti, who leads the state-run National Program to Rescue People Harmed by Trafficking Crimes.

Trimarco's attorney Jose D'Antona said he planned to seek the impeachment of the members of the court.

A disappointed Kirchner, a lawyer herself by training, said she hoped more “democratisation” could be brought to Argentina's court system.

“I thought that I would find her devastated,” the president said after her phone call to Trimarco. “But I found her more calm and centred than ever, and more determined to keep on fighting.”

As recently as Sunday, the president had honoured Trimarco, already a recipient with similar prizes in the United States and Canada, with a human rights award for her work.

Socialist Hermes Binner, who ran against Kirchner, told reporters “Susana Trimarco's lonely fight is in stark contrast with the impunity in the Marita Veron case.”

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.

“Let us build a commitment to change out of our outrage at this ruling,” urged Ricardo Alfonsin, another former presidential candidate, with the Radical Civic Union.

Ruling party Senator Beatriz Rojkes, who represents Tucuman and is married to its Governor Jose Alperovich, said she was “emotional and surprised” but added that “prostitution exists, and it will always exist.”

A saddened Trimarco told reporters: “we are not fighting against prostitution. We are fighting against the trafficking of women.” - Sapa-AFP