The law is “null and void,” presiding judge Steven Kavuma told the court, saying the process had contravened the constitution, as it has been passed in parliament in December without the necessary quorum of lawmakers.
Cheering gay rights activists celebrated the ruling, but supporters of the law said they would appeal at the Supreme Court.
“Justice prevailed, we won,” said lawyer Nicholas Opiyo, who led the challenge in the constitutional court.
“The retrogressive anti-homosexuality act of Uganda has been struck down by the constitutional court — it’s now dead as a door nail,” said Andrew Mwenda, one of 10 petitioners.
The law, signed by Uganda’s veteran President Yoweri Museveni in February, said that homosexuals should be jailed for life, outlawed the promotion of homosexuality and obliged Ugandans to denounce gays to the authorities.
US Secretary of State John Kerry likened the law to anti-Semitic legislation in Nazi Germany, and Western nations made a raft of aid cuts to Uganda’s government.
But homosexuality in Uganda remains illegal and punishable by jail sentences under previous legislation, which is expected to return after the court’s decision.
Critics have said Museveni signed the law to win domestic support ahead of a presidential election scheduled for 2016, which will be his 30th year in power.
Government spokesman Ofwono Opondo said the law — as it is on paper — remains valid, saying it had been struck down on a matter of procedure and not over its content. “The ruling has not nullified the anti-homosexuality law, it only ruled on the validity of the procedure in parliament,” he said.
But gay rights activists were celebrating.
“I am no longer a criminal, today we have made history for generations to come,” said Kasha Jacqueline, another petitioner and a prominent gay rights activist.
“I am officially legal,” said Frank Mugisha, another petitioner, but admitting despite his “celebration mood” the ruling was only the “beginning of a very long battle”. “The law has been struck on technicalities, so the big picture is still there,” he said.
Lawmakers could seek to reintroduce a bill to parliament, a potentially lengthy process, with the last such bill taking four years from introduction to the final vote.
Rights groups, who said the law triggered a sharp increase in arrests and assaults of members of the country’s lesbian, gay, bisexual and transgender community, welcomed the decision.
“We are pleased that this law cannot be enforced and entrench further abuses and discrimination,” said Maria Burnett from Human Rights Watch.
“Even though Uganda’s abominable anti-homosexuality act was scrapped on the basis of a technicality, it is a significant victory for Ugandan activists who have campaigned against this law,” Amnesty International said.
Homophobia is widespread in Uganda, where American-style evangelical Christianity is on the rise. Gay men and women face frequent harassment and threats of violence.
The 10 petitioners — including two Ugandan rights organisations — had also claimed that the law violated the constitutional right to privacy and dignity, as well as the right to be free from discrimination, and cruel, inhuman and degrading treatment.
But the judges stuck down the law based on only the first section, the issue of quorum, before any of those issues were discussed.
First Kenya took the law into its own hands. Then Uganda petitioned for its own justice. Again, I bemoan you, Nigeria. Will you ever know such justice?