Black lesbians in South Africa endure ridicule and abuse in schools, workplaces and churches, sometimes being accused of witchcraft, a Human Rights Watch (HRW) investigation has found.

"Lesbians and transgender men live in constant fear of harassment as well as physical and sexual violence," the watchdog group reported.

The research, We'll Show You You're a Woman, was based on interviews with 121 lesbians, bisexual women and transgender men over two years in the impoverished townships where most South Africans live.

Graeme Reid, director of HRW's lesbian, gay, bisexual and transgender rights programme, said: "It's a grim picture. It's a picture of fear and intimidation. A segment of South African society lives in terror and feels it has no one to turn to, including the police."

Same-sex marriage is legal in South Africa, and the country has some of the most liberal laws on sexual orientation on the continent. But cultural attitudes do not always match the constitution, approved in 1996.

One woman told HRW of a series of rapes by her cousin, her coach and her pastor. Another said a female cousin spiked her drink so that the cousin's boyfriend could rape her. A third said that, after a rape, "I really hated myself."

Raping a lesbian, HRW researchers found, can make a man a township hero. Attackers boast publicly of their crimes and declare to their victims: "We'll show you you're a woman," the report said. Such attacks are often referred to as "corrective rapes" in South Africa. Lesbians and others who do not fit the norm respond by avoiding being alone in public, trying not to attract men's attention, and hiding their sexual orientation, the report added.

The lead researcher, Dipika Nath, said lesbians and transgender men faced physical and verbal abuse that led to their dropping out of school, losing their jobs and becoming economically marginalised. Those who did not follow conventional patterns of dress and appearance were particularly vulnerable.

"Families, churches and schools – all the spaces of socialisation – are very often homophobic and transphobic," she said. "Churches very often become spaces that further the stereotypes: 'We don't want Sodom and Gomorrah'; 'We don't want witches'; 'These people don't belong in Africa.'"

Nearly all those interviewed by HRW said they were reluctant to approach the police for protection or to report crimes. Of the few cases of sexual or physical violence against lesbians that have been prosecuted, the significance of sexual orientation has been acknowledged in only one.

HRW called on South Africa's government to act against the attackers but also to tackle the problem at its roots. "What we really need is a sustained, large programme" that embraces education in schools and engages with religious leaders, Nath said.

A number of lesbians have been murdered, apparently because of their sexuality, in what activists believed should be classified as "hate crimes". Noxolo Nogwaza, 24, was stoned and stabbed to death and apparently raped in Kwa-Thema township in Gauteng earlier this year.

Nomboniso Gasa, a gender policy analyst, said the country needed to confront a culture of violence and homophobia that betrayed its democratic foundation. "This violence makes a farce of all that we claimed to achieve in post-apartheid South Africa in the constitution," Gasa said.

"If there was any other group that was targeted in the way LGBTI [lesbian, gay, bisexual, transsexual and intersex people] are targeted, South Africa would declare a crisis. This report lifts a veil of silence to make visible the realities South Africa would rather pretend do not exist."