Friends discovered the body of Eric Ohena Lembembe at his home in the capital, Yaounde, on Monday evening after he was unreachable for two days, the rights group said in a statement Tuesday.
One friend said Lembembe's neck and feet looked broken and that he had been burned with an iron.
Lembembe was among the most prominent activists in one of Africa's most hostile countries for sexual minorities. First as a journalist and later as executive director of CAMFAIDS, a Yaounde-based human rights organization, he documented violence, blackmail and arrests targeting members of Cameroon's gay community. He was also a regular contributor to the Erasing 76 Crimes blog, which focuses on countries where homosexuality is illegal, and he wrote several chapters of a book released in February on the global gay rights movement titled "From Wrongs to Gay Rights."
"Eric was an inspiring activist whose work was deeply appreciated by human rights activists in Cameroon and around the world," said HRW senior LGBT rights researcher Neela Ghoshal.
Lembembe is the most high-profile African gay rights activist to be killed since 2011, a year that saw the deaths of Uganda's David Kato and South African lesbian activist Noxolo Nogwaza.
Homosexuality is punishable by prison terms of up to five years in Cameroon, and the country prosecutes more people for gay sex than any other in sub-Saharan Africa, according to Human Rights Watch.
Lembembe was among the first activists in the country to mobilize following a sharp increase in anti-gay prosecutions beginning in 2005, said Yves Yomb, executive director of Alternatives-Cameroon, a gay rights group based in Cameroon's largest city, Douala.
"It is a big loss for our community, and we are a bit scared about what can happen to us," Yomb said. "He was one of the leaders of this community. So what can happen to the other leaders?"
Yomb said Lembembe was known as "Princess Erica" because of his designer clothes and his gracious way with visitors, offering them fresh juice and an attentive ear.
But he was also a tenacious journalist and activist, Yomb said.
"He was very, very, very active in the fight for LGBT rights in Cameroon," Yomb said. "In his work as a journalist, when he wanted to have information he would call you until you gave him the information that he needed."
Just two weeks ago, Lembembe spoke out against a recent spate of break-ins at organizations advocating for gay rights in Cameroon.
"There is no doubt: anti-gay thugs are targeting those who support equal rights on the basis of sexual orientation and gender identity," Lembembe said in a statement on July 1. "Unfortunately, a climate of hatred and bigotry in Cameroon, which extends to high levels in government, reassures homophobes that they can get away with these crimes."
Ghoshal told The Associated Press that friends grew concerned for Lembembe on Saturday when he uncharacteristically missed a meeting.
On Monday, they visited his house and found the door locked from the outside, though they could see Lembembe's battered body through a window.
Law enforcement officials in Yaounde could not be reached Tuesday. Ghoshal said officials had transported the body to a hospital and begun taking statements.
The precise motive for the killing was not immediately clear, but Ghoshal said Lembembe's friends indicated he had no enemies.
"As far as they know he didn't have any other disputes, relationship disputes or money matters, so they attribute this to a likely homophobic killing, but that's just speculation at this point," Ghoshal said.
She added that while the initial steps of an investigation appeared to be underway, it was unclear whether officials would follow through.
"What worries us is that in the previous incidents of homophobic threats or violence, the police have done the basics but it's never gone beyond taking statements," she said. "It remains to be seen whether the police will carry out a serious investigation into this matter."
When News Breaks Out, We Break In. Minute by Minute Report on Cameroon and Africa