The Human Rights Foundation has strongly criticised Mariah Carey for entertaining Angola's veteran President Jose Eduardo dos Santos, a "notoriously corrupt" dictator in Uganda.
Carey performed for two hours on Sunday at a gala for the Angolan Red Cross, which was sponsored by mobile phone company Unitel.
The gala raised at least $65,000 for the charity, according to the state news agency Angop.
The 43-year-old singer posed with the president, his wife and daughter at the event, where she said she was "honoured to share this show with the President of Angola."
Carey was reportedly paid $1 million (£610,100) for her performance.
"Mariah Carey can't seem to get enough dictator cash" said The Human Rights Foundation leader Thor Halvorssen, referring to an earlier show in which Carey performed for former Libyan ruler Muammar Gaddafi's son.
"She goes from private performances to public displays of support and credibility for one of Africa's chief human rights violators and most corrupt tyrants.
"It is the sad spectacle of an international artist purchased by a ruthless police state to entertain and whitewash the father-daughter kleptocracy that has amassed billions in ill-gotten wealth while the majority of Angola lives on less than $US2 a day."
All I Want for Christmas singer Carey made a public apology for the gig in Lybia five years ago.
At the time, the star said she was naïve and unaware of whom she was booked to sing for.
"I feel horrible and embarrassed to have participated in this mess," Carey said.
"Ultimately we as artists are to be held accountable. Going forward, this is a lesson for all artists to learn from.
"We need to be more aware and take more responsibility regardless of who books our shows."
However, Carey is not the first star to come under fire for getting paid to appear at events involving a controversial ruler. Jennifer Lopez, Kanye West and Beyonce have also been criticised for similar appearances.
Angola is Africa's second-biggest oil producer after Nigeria but it is ranked by Transparency International as the 25th most corrupt of the world's 177 countries.
Santos has been President of Angola since 1979; and he has been accused of corruption and crackdowns several times. Mr Santos has also reportedly ordered the deaths of many politicians, journalists and activists who oppose him.