Amnesty International and the Open Society Justice Initiative have urged the Haitian authorities not to drop a rights case against former ruler Jean-Claude “Baby Doc” Duvalier.
The international human rights groups said Jean-Claude Duvalier “must not evade justice” for alleged crimes against humanity.
In January 2012, a judge ruled that the alleged abuses had expired under Haiti’s statute of limitations.
An appeal hearing against that decision is due to begin on Thursday.
Jean-Claude Duvalier unexpectedly returned to Haiti in 2011 after 25 years in exile, prompting the Haitian authorities to open an investigation into crimes allegedly committed during his 1971-86 rule.
Baby Doc Duvalier has denied all the accusations against him.
A judge decided that he should stand trial for embezzling public funds but ruled that the statute of limitations had run out on charges of murder, arbitrary arrest, torture and disappearances.
Alleged victims and their relatives have appealed against this ruling.
A first hearing was postponed when Jean-Claude Duvalier failed to appear in court on 31 January. He has been ordered to attend Thursday’s hearing.
International law requires that he should stand trial for alleged crimes against humanity, the Open Society Justice Initiative said.
Amnesty International has also argued that such crimes are not subject to a statute of limitations.
“With the case of Jean-Claude Duvalier, it is the whole credibility of the Haitian justice system which is at stake,” Amnesty said.
“Only by respecting the procedures in the appeal case, including thoroughly examining all evidence and hearing all the victims, will the court be able to demonstrate the professionalism and independence of the Haitian justice system.”
Jean-Claude Duvalier was just 19 when he inherited the title of president-for-life from his father, Francois “Papa Doc” Duvalier, who had ruled Haiti since 1957.
Like his father, Jean-Claude Duvalier relied on a brutal militia known as the Tontons Macoutes to control the country.
In 1986 Jean-Claude Duvalier was forced from power by a popular uprising and US diplomatic pressure, and went into exile in France.