A 35-year-old woman walked through a field in northern Mozambique, near where a group of teenage boys were undergoing their ritual circumcision into adulthood.

Accusing her of trespassing on sacred ground forbidden to women, the traditional leader meted out his punishment: He ordered 17 of his young initiates to gang rape her.

Four of the youths were arrested, but no trial date has been set, Delfino Jose, from Mozambique's unit for crimes against women told AFP. In Mozambique, many court cases simple fall off the radar and never go to trial.

The case has ripped open a searing debate about women and tradition, in a country lauded for advancing women in politics. The previous prime minister and current parliament head are women, as are 39 percent of lawmakers. Mozambique won the African Gender Award for promoting women in 2009.

But this is just window-dressing in a nation where women suffer brutal attacks, said activist Gilberto Macuacua.

In January a man in central Mozambique cut off his wife's arm with a machete when she confronted him about his affair with her cousin. Last year a man pierced his wife's vagina with a bayonet, then sewed it shut.

"Men still aren't ready to see women as people with the same rights as men," Macuacua said. "There is little or no knowledge of human rights, especially women's rights."

"There are many women in parliament, but most don't exert much pressure," he added.

Award-winning health journalist Pedro Nacuo justified the gang rape in a column, saying the woman had repeatedly trespassed on purpose.

Hospital staff treated her like an attempted suicide case, assuming she knew the consequences of walking in the area, he said.

"The nurses... treated her as they would treat someone who deliberately steps in front of a car to be run over," he wrote in the state-run Noticias newspaper.

"No one condemns, morally or traditionally, the punishment applied to this lady, although severe, because our institutions are made up of people who know how to deal with these issues," he said.

Women's rights organisation Muleide defended the mother of five, saying she was walking to her crops and didn't realise it was close to an initiation site.

"This case recalls the collective rape of a young woman ordered by a tribal council in a rural area of Pakistan in 2002," said a coalition of six advocacy groups in a statement.

"Contrary to the journalist's conservative, sexist and Talibanist views, authorities have to intervene in this case," the coalition said following Nacuo's statements.

Nacuo declined to comment to the "uninitiated", but has branded his critics as lackeys of foreign donors.

Mozambique in 2009 passed a law on domestic violence. In a survey three years earlier, more than half the respondents had been victims of physical or sexual violence. Experts say the real figure is probably higher.

"It's unbelievable, because we have many formal instruments which guarantee equality, but then we encounter resistance to implement them, incompetence and complicity of the institutions that should do this," said anthropologist Maria Jose Arthur from Women and Law in Southern Africa.

"There's a lot of resistance to administer this law because people have prejudices," she told AFP.

Macuacua agreed: "When a woman does speak, it's a type of threat. Women cannot speak."