A Ugandan pastor who screened same-sex pornography in a church to try to bolster support for proposed anti-homosexuality legislation has been condemned by gay rights groups.

Martin Ssempa, one of the main backers of a bill that would impose the death penalty for some offenders, aired the explicit slideshow to several hundred people during a church service in Kampala yesterday. Explaining his decision to display the images, the evangelical preacher said it was necessary to educate people "about what homosexuals do".

He told the BBC's Network Africa programme: "In Africa, what you do in your bedroom affects our clan, it affects our tribe, it affects our nation."

While Ssempa represents the extreme end of widely held homophobic views in Uganda, he does carry strong influence. On his website he describes himself as "consultant to the government of Uganda", and his congregants yesterday included David Bahati, the MP who submitted the private member's bill to parliament last year.

The porn stunt caused anger among gay activists. South African-based gay rights group Behind the Mask described it as "twisted homophobic propaganda".

Frank Mugisha, the chairman of Sexual Minorities Uganda, said Ssempa should be arrested for promoting pornography – and questioned his religious values. "He is showing these images in a church. What does he stand for?"

In neighbouring Kenya, where anti-gay sentiment is also strong, Peter Njane of gay support group Ishtar said that by showing pornography Ssempa was trying to imply that being gay was "just about sex".

In fact, that is one of the main arguments of the pastor and politicians such as Bahati, who argue that homosexuality is a lifestyle choice. Although gay sex is already illegal in Uganda under colonial-era laws, they have whipped up public sentiment by insisting that tougher measures are needed to punish homosexuals who they say are "recruiting" children at schools.

The legislation calls for life imprisonment for anyone caught having gay sex, with the death penalty for repeat offenders or those with HIV. But the proposals have generated much opposition abroad, with the governments of Britain, the US and Canada, all significant donors to Uganda, leading the condemnation.

The pressure appears to have worked. Last month President Yoweri Museveni broke his silence on the bill, saying it did not necessarily reflect government policy and "foreign policy interests" would be considered before it was put to a vote.