More than 700 million women in the world today were married before their 18th birthday and one in three of these was women was married before the age of 15, a new report says today.
Besides the risks of child marriage, an estimated 30 million girls are at risk of female genital cutting in the next decade.
Adolescent girls are more likely to experience sexual violence and other forms of violence than boys, the report finds.
The issues are exacerbated because 2.6 billion girls and women live in countries where marital rape is not a criminal offence.
Rebeca Zakayo Gyum, a girls' rights advocate from Tanzania, says in the report: "We need to finally free girls from all the barriers they face, and empower them to stand up for their rights and make their voice heard. This cannot wait."
Every last girl must be free to live, free to learn and free from harm, she says.
"This is only possible if we collectively pledge to advance girls' rights to access education, to access information on sexual and reproductive health rights, and to make their voices heard."
The issue is not religious but social.
Alice Klein, media manager at Save the Children, told Christian Today: "In a lot of countries in Asia which are Muslim it is in issue. But similarly in a lot of countries in sub-Saharan Africa which are Christian it is an issue. So it is social rather than a religious issue."
She said child marriage comes out of poverty and tradition. "But fundamentally it is about the way society values girls, or rather, under-values girls. It is really about empowering girls to get an education and be supported to make life choices themselves."
A lot of change is happening, but not fast enough. Along with rapid population growth, this means that the goal of ending child marriage by 2030 is unlikely now to be achieved.
One way to help is through charities and agencies that with families and communities to persuade them not to marry their daughters off. Another way to help is to get more girls into education. "While girls are in schools, they are less likely to be in marriages," said Klein.
The report quotes Nobel Peace Prize laureate, Malala Yousafzai, who now lives in Birmingham having survived an attempt by the Taliban in Pakistan to shoot her dead on her school bus: "I don't want to be remembered as the girl who was shot. I want to be remembered as the girl who stood up."
The report says: "The kidnapping of the Chibok girls in northern Nigeria and the sexual enslavement of Yazidi girls in Iraq are motivated by the same pernicious notion – that girls should not be free to learn or to make decisions about their lives.
"Other girls – such as those who have been trafficked across the Mediterranean, or who are forced to live in a brothel in Bangladesh – are subject to the most extreme violations of their rights because of another noxious idea: that girls are tradable commodities."
At the same time, the report continues, different forms of disadvantage – poverty, disability, membership of a minority ethnic or religious group, and geographical location – overlap to put particular groups of girls at greater risk of disempowerment and exclusion.
The report notes that girls in Sierra Leone are among the most likely in the world to be child mothers.
In Sierra Leone, 28 per cent of girls aged 15 to 19 have already had a child or are pregnant. Teenage girls account for 40 per cent of all maternal deaths in the country.
"Girls are being forced to turn to transactional sex as an economic coping strategy, particularly in the wake of the Ebola crisis, and young mothers are blamed, stigmatised and often excluded from their communities," the report says.
Save the Children is running a fundraising campaign to help "set girls free".
The appeal quotes the story of Khadra, who when she was 15 was forced to marry a man who was over 30 years old. She was in her first year of secondary school but she had to leave. "I wanted to continue with school and go to university and become a doctor," she says.
Instead, her husband beat her. By the time she left him, she was already pregnant. After she gave birth, she was unconscious for three days. Her body suffered in labour and she is still in pain.
She says that girls should not be forced to get married: "Girls who give birth at a young age face a lot of health complications."
Another case study is of Waafa, (not her real name), now aged 19, who moved to Lebanon from Homs, Syria a few years ago.
At that time she was 16 and pregnant. She later learned her husband, who had sent her to Lebanon for safety, had died.
She was married at 16 because her parents were worried that they could not protect her from armed groups.
Fighting had escalated and several unmarried girls had been assaulted and forced to marry men from armed groups.
Wafaa said that if certain suitors were rejected, there could be violent repercussions against the families.
Both she and her mother described the lawlessness and chaos that led to the decision for her to marry early. Were it not for the war, she would have stayed in education.
Kamla (not her real name), 14, was "married" at three months old. She only found out about this when she was ten and arrangements had begun to be made for her to move to her husband's house
She had dropped out of school at 10 and her grandmother thought she was old enough to live with her husband because she was so tall.
She went on hunger strike to persuade her parents to let her go to school and by the third day they took her to the school.
She does not know what will happen with the agreement but she hopes to continue her studies. She has excelled at sports and wants to be in the police when she is older to help end violence against women.