Post-apartheid South Africa has one of the world's most liberal constitutions. It is the only country in Africa - a continent where homosexuality is widely vilified - to enshrine equal rights for gays and lesbians and to recognise citizens' rights to enter same-sex marriages.

But the constitutionally enshrined civil liberties of Soweto lesbian couple Sizakele Sigasa and Salome Masooa did not prevent their rape, torture and murder last week in an apparent execution-style homophobic attack.

Sigasa, 34, a prominent lesbian rights activist, was shot six times in her head and collarbone. Her underpants had been used to tie her hands behind her back, and her ankles had been tied together with the laces from her own trainers. Masooa, 23, had been shot through the back of her head.

The pair had left a Soweto party to take a friend home on the night of the attack. Their bodies were later discovered near their car by a jogger. Alhough no arrests have been made, in a country where daily murders are commonplace, gay rights organisations said the killings were driven by "lesbophobia".

The Joint Working Group, the umbrella organisation for South Africa's gay and lesbian associations, said the murders were part of a growing epidemic of hate crimes.

In the past two months there have been two other murders of lesbian women in black townships. Simangele Nhlapo, a member of a support group for women living with Aids, was raped and murdered; her two-year-old daughter was raped and left with both her legs broken. In another incident, 16-year-old Madoe Mafubedu, living openly as a lesbian, was raped and repeatedly stabbed until she died.

While many prominent white South Africans live openly and safely in gay and lesbian relationships, Zanele Muholi, community relations officer with a prominent lesbian rights group, said: "It is being both black and gay which is problematic.

"I have recorded 50 rape cases, dating back 10 years, involving black lesbians in townships. Rape and violence against lesbians is common. The problem is largely that of patriarchy The men who perpetrate such crimes see rape as curative and as an attempt to show women their place in society."

The icon of South Africa's black lesbian movement is Zoliswa Nkonyana, who was clubbed and kicked to death aged 19 in 2006 by a group of about 20 young men in the Cape Town township of Khayelitsha for being an openly practising lesbian.

Among the prominent people Sigasa had campaigned against was former vice-president Jacob Zuma, who has just fathered his 18th child by a ninth woman, and who prior to his trial for raping his niece last year denounced gay people as "a disgrace to the nation and to God".

But while gays and lesbians have constitutional protection in South Africa, elsewhere on the continent being gay is considered un-African, un-Christian, anti-family and a product of witchcraft.

From Uganda, where homosexuality is punishable by life imprisonment, to Zimbabwe, where President Robert Mugabe dismisses gays as "lower than pigs and dogs", homophobia traps gay people in dangerous, closeted lives.

In Zimbabwe, Tracy Mhara, a 33-year-old lesbian who lives in the capital, Harare, travels the 90 miles to visit her family's rural home accompanied by a married male friend who she introduces as her husband-to-be to cover her tracks. When asked why he has not paid the customary "lobola" - a payment from a prospective husband to the bride's family of a dozen or more cows - he smiles and pleads poverty.

Zimbabwe's first post-independence ceremonial president, the Reverend Canaan Banana, died a publicly disgraced figure after a high-profile sodomy conviction. Banana, a Methodist minister and father of four, was revealed in his 1999 trial as a closet homosexual who habitually abused his young male aides, using drugged soft drinks and promises of career advancement to secure sexual favours. Banana was briefly jailed after being sentenced in 2000. He died in November 2003.

Many persecuted Zimbabwean gays have fled to Britain, including Kudah Samuriwo, one of the country's most outspoken homosexuals, who has become a drag performer on the London theatre circuit with his show The Queen of Africa. One of his favourite jokes goes: "I don't know what Mugabe has against pigs and dogs. He must have had the worst sex ever with them."

Samuriwo said he was raped at the age of 14 by his uncle, a soldier, after the man returned from Mugabe's military crackdown on the minority Ndebele people of western Zimbabwe.

Samuriwo's show charts his personal story, including Mugabe's oppression of the gay community, with homosexuals repeatedly bribed, detained, beaten and raped by the authorities. And he intends to take his show back home to Zimbabwe one day as part of a new liberation struggle. "After all," he said, "a queen must protect her subjects, even if the president refuses to do so."