A Kenyan gay man wears a mask to preserve his anonymity as he protests against anti-gay moves in Uganda outside the Uganda High Commission in Nairobi, Kenya on 10 February The bill would allow life imprisonment for acts of "aggravated homosexuality" and make it a crime not to report gay people
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Uganda's president has signed into law a bill toughening penalties for gay people and criminalising those who do not report them.
A government spokesman said President Yoweri Museveni wanted to assert Uganda's "independence in the face of Western pressure".
US President Barack Obama has cautioned the bill would be a backward step.
The president had previously agreed to put the bill on hold pending US scientific advice.
Homosexual acts are already illegal in Uganda.
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In December a gay rights campaigner spoke of her fears about the legislation
The new law punishes first-time offenders with 14 years in jail, and allows life imprisonment as the penalty for acts of "aggravated homosexuality".
The law makes it a crime not to report gay people - in effect making it impossible to live as openly gay.
It criminalises the "promotion" and even the mere "recognition" of homosexual relations "through or with the support of any government entity in Uganda or any other non-governmental organisation inside or outside the country".
Lesbians are covered by the bill for the first time.
The bill originally proposed the death penalty for some homosexual acts, but that was later removed amid international criticism.
Earlier government spokesman spokesman Ofwono Opondo told Reuters news agency Mr Museveni wanted to sign the bill in front of the international media "to demonstrate Uganda's independence in the face of Western pressure and provocation".
The sponsor of the bill, MP David Bahati, insisted homosexuality was a "behaviour that can be learned and can be unlearned".
"Homosexuality is just bad behaviour, that should not be allowed in our society," he told the BBC's Newsday programme.
But a gay rights activist in Uganda told the programme that he was "very scared" about the new bill.
"I didn't even go to work today [Monday]. I'm locked up in the house.
"And I don't know what's going to happen now. I'm talking to all my activists on the phone. And it's the same, they are all locked up in their houses. They can't move out. They are watching their back to see what happens."
Mr Museveni's renewed determination to sign the bill is an apparent U-turn from a recent pledge to hold off, pending advice from the US.
In a statement, Mr Museveni had said: "I... encourage the US government to help us by working with our scientists to study whether, indeed, there are people who are born homosexual.
"When that is proved, we can review this legislation."
President Obama described it as "more than an affront, and a danger to, Uganda's gay community. It will be a step backwards for all Ugandans".
He warned it could "complicate" Washington's relations with Uganda.
The US provides a reported $400m (£240m) in annual aid to Uganda.
In South Africa, former archbishop Desmond Tutu said he was disheartened by President Museveni's apparent change of stance.