BREDASDORP, South Africa (Reuters) - Hundreds of South Africans chanting "enough is enough" gathered on Friday at a building site where a 17-year-old girl was mutilated and left to die after being gang-raped.

Their tribute was testimony to how the killing of Anene Booysen has stirred outrage in a country where many people have become oblivious to rampant sex crimes and violence.

People marched in a procession to the site in the sleepy town of Bredasdorp, 130 km (80 miles) east of Capte Town, where they placed flowers and candles by a simple wooden cross.

Booysen was found by security guards lying only a short distance from her house after partying at a nearby bar last Friday evening. She later died in hospital.

Her foster mother, Corlia Olivier, recounted the moment when she saw her daughter dumped amid the gravel and grass, her stomach slit open down to her genitals.

"I heard her saying ‘Mommy help me, Mommy help me' and I rushed over...and just saw her guts hanging out," Olivier told reporters, tears welling up in her eyes.

The incident and the public response has echoes of the gang rape and murder of a 23-year-old student on a New Delhi bus last year, which caused a national and international outcry.

President Jacob Zuma expressed shock and outrage, calling for the harshest possible sentences for the killers and a concerted campaign "to end this scourge in our society".

In Geneva, U.N. Human Rights chief Navi Pillay said South Africa must tackle a "pandemic of sexual violence" and she was concerned about low conviction rates for rapists.

"There is a need for very strong signals to be sent to all rapists that sexual violence is absolutely unacceptable and that they will have to face the consequences of their terrible acts. The entrenched culture of sexual violence which prevails in South Africa must end," said Pillay, herself a South African.

South Africa has the highest number of reported rapes per head of population of any Interpol member country, with more than 64,500 reported in 2011/2012.

Even when suspects are caught, only 12 percent of cases end in conviction. But sexual crimes seldom cause little public concern beyond some soul-searching editorials and anguished radio phone-ins.

The Womens' League of the ruling African National Congress is trying to mobilise the public into similar action to the protests against anti-female violence that took place in India after the New Delhi attack.

On Friday, Cape Town radio station KFM broadcast a "bleep" every four minutes as a reminder to listeners that another South African woman will, on average, have just been raped.

DEATH PENALTY?

At the Bredasdrop site, religious leaders and politicians linked arms with Booysen's relatives as they sang hymns and laid a wreath by the cross, adorned with a pink ribbon.

"I still hear her footsteps," Olivier said as a stream of well-wishers arrived to offer condolences.

Maree Louw, commander of the local police station, said the murder was one of the worst cases she had seen in a long career.

"The brutality and the slaughter of this young teenager is beyond belief," Louw told Reuters.

Like many towns in South Africa's Western Cape, Bredasdorp, with a population of 35,000 people, has problems with drug and alcohol abuse but, Louw said, most people would go to bed at night with their back doors open and windows unlocked.

Booysen managed to reveal the name of one of the attackers, a family friend, before dying. Three men in their early 20s have been arrested and are expected to appear in court on Tuesday on charges of rape and murder.

They face the prospect of life in prison if convicted.

Under a constitution drawn up after the end of apartheid in 1994, Nelson Mandela's "Rainbow Nation" abolished the death penalty. Some in Bredasdorp wish that were not the case.

"This crime was very sadistic and deserves the death penalty," said mother-of-three Sophia Europa. "What they did was worse than anything done to an animal."

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