Archbishop Desmond Tutu has condemned Uganda's proposed law against homosexuality, saying there is no scientific or moral basis ever for prejudice and discrimination – and accusing the Ugandan president of breaking a promise not to enact the law. The new law would extend the prohibitions and penalties in a country where homosexuality is already a crime, to include acts such as "suggestive touching" in public.

President Yoweri Museveni had first said that he would not sign the legislation, then that he would do so after seeking scientific advice, and at the weekend that he would delay it pending more advice.

The proposed law has drawn harsh criticism from US president Barack Obama and former president Bill Clinton. The US warned that such a move could "complicate" approximately £240m in annual aid to Uganda. In a statement Tutu said: "When President Museveni and I spoke last month, he gave his word that he would not let the anti-homosexuality bill become law in Uganda. I was therefore very disheartened to hear last week that President Museveni was reconsidering his position."

Tutu equated discrimination against gay people with the horrors of Nazi Germany and apartheid-era South Africa.

"We must be entirely clear about this: the history of people is littered with attempts to legislate against love or marriage across class, caste, and race. But there is no scientific basis or genetic rationale for love. There is only the grace of God. There is no scientific justification for prejudice and discrimination, ever. And nor is there any moral justification. Nazi Germany and apartheid South Africa, among others, attest to these facts."

The retired archbishop recalled apartheid-era police raids: "In South Africa, apartheid police used to rush into bedrooms where whites were suspected of making love to blacks. They would feel if the bed sheets were warm, crucial evidence to be used in the criminal case to follow. It was demeaning to those whose 'crime' was to love each other, it was demeaning to the policemen – and it was a blot on our entire society."

Tutu went on to plead with Museveni to use the debate to strengthen the culture of human rights and justice in Uganda, and clamp down on sexual exploitation rather than orientation. "To strengthen criminal sanctions against those who commit sexual acts with children, regardless of gender or sexual orientation. To strengthen criminal sanctions against all acts of rape and sexual violence, regardless of gender or sexual orientation," he said. And, if needs be, to strengthen criminal sanctions against those involved in commercial sexual transactions – buyers and sellers regardless of gender or sexual orientation. Tightening such areas of the law would surely provide children and families far more protection than criminalising acts of love between consenting adults.

Same-sex relations are criminal in 36 of 55 African countries, and carry the death penalty in some.