Uganda's President Yoweri Museveni has signed one of the world's toughest anti-LGBTQ laws, including the death penalty for "aggravated homosexuality", in defiance of Western condemnations and potential sanctions from aid donors.

Same-sex relations were already illegal in Uganda, as in more than 30 African countries, but the new law goes much further.

It imposes capital punishment for some behaviour including transmitting a terminal illness like HIV/AIDS through gay sex, and stipulates a 20-year sentence for "promoting" homosexuality.

"The Ugandan president has today legalised state-sponsored homophobia and transphobia," said Clare Byarugaba, a Ugandan rights activist. "It's a very dark and sad day for the LGBTIQ community, our allies and all of Uganda."

She and other activists have vowed a legal challenge to the law, which Museveni was shown signing at his desk with a golden pen in a photo tweeted by Uganda's presidency.

The 78-year-old leader has called homosexuality a "deviation from normal" and urged lawmakers to resist "imperialist" pressure.

A less restrictive 2014 anti-LGBTQ law was struck down by a domestic court on procedural grounds, after Western governments had initially suspended some aid, imposed visa restrictions and curtailed security cooperation.

Uganda receives billions of dollars in foreign aid each year and could now face another round of sanctions.

The bill's sponsor Asuman Basalirwa told reporters that parliament speaker Anita Among's U.S. visa was cancelled after the law was signed....