These sexual initiations begin once menstruation starts and sometimes involve sticks being inserted inside the girls, Persilia Muianga of international aid agency World Vision said.
She added that some mothers force young daughters to sleep with a man in the belief this can bring on menstruation. Anglican priest Jackson Jones Katete said initiations in Zambia happen among girls between the ages of eight and 13, and may involve girls being cut by women for not performing sexual movements correctly.
"You ... pay these (elderly) women to do this torturing to your child," he said, adding that men do not want to marry girls unless they have been initiated.
"Immediately the girls come out of the camp, they are saying ... you are now ready for sex. And then the men come ... and then they begin to do the betrothals."
The training, which can last a week and is shrouded in secrecy, also teaches girls about hygiene, domestic duties and how to conduct themselves in the community, Muianga said, adding that community leaders fine parents if they do not take their daughters to the initiations.
Muianga, a child protection expert, said the sexual age of consent in Mozambique is 12 and many girls have babies very early, putting their lives at risk.
Serious childbirth injuries such as fistulas are a big problem because so many girls have babies before their bodies are ready, she told the Thomson Reuters Foundation during the conference in the Moroccan city Casablanca.
Nearly half of girls in Mozambique and more than 40 percent in Zambia are married before they turn 18, even though child marriage is illegal in both countries.
Bride prices paid to the girl's family drive early marriage in poor rural areas, Muianga said.
She said World Vision is training church leaders to tackle issues around early initiations and child marriage, and will help develop a similar initiative for Muslim communities.
Katete, who is director of the Anglican Street Children's Program in Zambia, said church leaders carry great authority in his country and have a role to play in addressing initiations and child marriage with their congregations.
He added that keeping girls in school is crucial for fighting early marriage, but most rural communities do not have schools nearby and teachers in these regions are usually men, which sends girls the signal that only boys deserve education.
"We are now saying that you should build schools in villages and have female teachers there as well who can act as role models."
The 3 day conference ending Thursday is hosted by Girls Not Brides, a global partnership committed to eradicating child marriage which affects some 720 million women worldwide.