It is a country that can seem inured to violence from sheer familiarity. But this was a crime so shocking that even South Africa has begun to search its soul.

A disabled 17-year-old girl, said to have the mental capacity of a five-year-old, was allegedly gang-raped by seven men and boys in an assault that came to light only when a mobile phone video of the brutal attack went viral online.

Outrage has been compounded by reports that the girl was missing from her home in the sprawling Soweto township for three weeks, yet no one raised the alarm, and police were only prompted to act after the video appeared.

There were angry scenes on Thursday when the seven suspected rapists appeared in a Johannesburg court. Protesters outside the building waved posters that read: "Cut their penis – no bail," "Let them rot in jail," and "Done with rapists."

Heinous acts of violence are all too unexceptional in South Africa, where nearly 16,000 murders and more than 66,000 sexual offences were recorded by police last year. But the stark details of this case have prompted a rare bout of introspection, particularly about the status of women.

"This is a story of complicity, gender violence and neglect in Soweto and is, in all likelihood – to varying degrees – representative of what so very often happens to girls and women at the hands of this nation's men," said a pointed story on page one of the Times newspaper.

"The complicity lies in the fact that a community knew this teenager, a child really, and must have known she had previously disappeared for weeks on end. She had apparently been raped several times since 2009. But no one seems to have found it necessary to report this to the authorities."

The article added: "But perhaps most tragic of all is the way that the 17-year-old was discovered yesterday.

"As police officers moved through the township, using loudhailers to ask community members to come forward with information about the missing girl, she was brought from a 37-year-old man's one-roomed home.

"He claimed she was his 'girlfriend' and that she had arrived at his house on Monday. Hungry, dazed and confused, the girl was yesterday unable to recall her whereabouts."

The mobile phone video, filmed on 21 March, reportedly shows the girl being raped in an open field, screaming "You are forcing me," and pleading for her assailants to stop. They ignore her cries, joking and laughing as they take turns. Eventually one offers her two rand – about 16p.

Under the front-page headline "A nation's shame," the Star lamented: "We are a nation of heroes; of Mandelas, Tambos, Luthulis, Bikos, De Klerks and Tutus, South Africans who won the world's praise for their courage and humanity. Today, though, we have tarnished their legacy – and the countless millions of decent South Africans who find this news as abhorrent as we do."

Politicians joined the condemnation. The women's league of the governing African National Congress said words could not describe its "outrage, anger and disgust". It demanded: "When does it become acceptable amongst a group of peers to rape a girl and laugh about it? It just makes one sick to the stomach."

South Africa boasts one of the most progressive constitutions in the world, enshrining women's rights. Yet activists estimate that more than one in three South African women will be raped in their lifetime.

Lisa Vetten, executive director of the Tshwaranang Legal Advocacy Centre, which campaigns to end violence against women, said: "All the causes of rape you will find in South Africa."

She listed causes including a "dysfunctional" criminal justice system, a historical culture of violence in which such acts are tolerated, a very unequal relationship between men and women, a lack of adequate childcare, which results in the neglect of boys, a high rate of male unemployment and inadequate aftercare services for perpetrators and victims.

Mbuyiselo Botha, a spokesman for the Sonke Gender Justice Network, said: "Men in our country have a sense of impunity. They rape because they can and because they can get away with it. Women are seen as fair game."

Critics have accused the president, Jacob Zuma, a traditional Zulu polygamist due to marry his fourth wife this weekend, of failing to lead the way. Botha said: "Males in leadership in our country have not, by their words or actions, spoken out loudly. Our president has not said: 'Not in our name.'"