Girls and women who speak out against female genital mutilation are being attacked, abused and harassed by members of their communities determined to keep the crime a secret.

The Guardian has spoken to women who have received death threats, been publicly assaulted and who have had to move house after speaking out about FGM, which involves cutting away some or all of a girl's external genitalia and can include sewing up the vagina. It is mostly carried out on girls some time between infancy and the age of 15.

Nimko Ali, a 29-year-old British-Somalian, was taken to Somalia for the procedure when she was seven. "I never told anyone I had FGM, not even my best friend, because I saw what happened to women in the UK who did speak out and saw it as a warning sign," said Ali, who has set up a group called Daughters of Eve to campaign against the procedure.

"I only decided to go public very recently after seeing other girls put themselves in danger by speaking out. The weeks afterwards were the most horrifying of my life. I lost friends – one even offered to kill me for £500."

The abuse, Ali said, had not waned. "A man recently threw a liquid in my face in the street . I was terrified; I thought it was acid. He was screaming that I was 'a slag' and needed to learn some shame."

FGM is not condoned by any religion. It is illegal in the UK to carry out the procedure, take a British citizen abroad to have the operation, or assist in carrying out FGM abroad, whether or not it is against the law in that country.

But although almost 160 incidents were recorded in the 2008-09 British crime survey, there have been no convictions since it was criminalised in 1985. Although FGM is incorporated into child protection, at present no data is collected on the number or type of social-work cases involving it in the UK.

Efua Dorkenoo, a director at Equality Now, regularly receives death threats aimed at stopping her campaigns against FGM. "I'm told my offence in speaking out is greater than that of Salman Rushdie and that I should die," she said.

"Any woman or girl who speaks out against FGM is in very serious danger from extended members of their family, their neighbours and from their community, especially from so-called gatekeepers of their community who control and harass them if they raise their voices.

The intimidation is extreme. Girls and women are physically attacked in the street and followed at night. The windows of their houses are broken. They receive anonymous phone calls from men shouting intimidation and threats.

One woman was pushed to the ground and kicked – she had a child who was threatened too, and she ended up having to move house.

"You can't speak out against it without risking your life. I'm aware of three young girls who are currently in care for this very reason."

Dorkenoo says the backlash against women who speak out is getting more extreme. "It's getting worse for young girls because social media means they can be threatened and harassed by people outside of their community, including by family members back in Africa who are told what they're doing."

The first and only major piece of FGM research at a national level was in 2007 by the charity Forward, in collaboration with the London School of Hygiene and Tropical Medicine and the department of midwifery at City University, which was funded by the Department of Health.

The research, which used the 2001 census, found there were at least 66,000 women with FGM estimated to be living in England and Wales.

It identified around 21,000 girls aged eight or younger at high risk of FGM. It also found that more than 11,000 girls aged nine or over had a high probability of already having suffered FGM.

Muna Hassan, an 18-year-old member of the charity Integrate Bristol, a charity that helps young people from other countries and cultures, has suffered for her outspoken support of the group's campaign against FGM. "Men harass and intimidate us girls all the time," she said.

"We made a film about FGM called Silent Scream and they spread rumours that we were being paid to make a pornographic film. They rang our fathers anonymously and said we were humiliating our families in public.

"It horrified our parents and quite a few girls weren't allowed to do the project any more because of it.

"These are people who promote themselves as community leaders and elders. The scary thing is that these are the people that councillors and politicians go to when they want to discuss community issues."

Last week, prosecutors and police announced that they were to reopen investigations into six alleged FGM incidents between 2009 and 2012.

A separate inquiry is under way into an alleged conspiracy to carry out FGM on a girl in London. An eighth case, in which the Met police say they have clear evidence, is being considered by prosecutors.