Chad's ex-President Hissene Habre has gone to court to challenge his conviction for crimes against humanity.
His lawyers argue that the trial by a special court jointly set up by Senegal and the African Union was marred by procedural errors.
They also say the defendant's rights were not respected.
Mr Habre was the first former leader to be convicted by an African Union-backed court - his case was seen as a landmark for the continent.
The former president ruled Chad from 1982 until he was overthrown in 1990 and fled into exile in Dakar, the capital of Senegal.
He was sentenced to life in prison after being convicted of rape, sexual slavery and ordering mass killings.
Two months after Habre's conviction, the court ordered him to pay up to $33,000 (£27,000) to each of his victims or their surviving relatives.
Prosecutors hope to find a way to locate and seize Habre's assets for the compensation of his victims.
An estimated 40,000 people in total were killed and many more kidnapped, raped or tortured under Habre's regime.
As his appeal trial opened on Monday, one of his lawyers, Mbaye Sene, told the AFP news agency:
"We were motivated to appeal by the violations of the law and [the rights] of the defence and procedural errors."
Habre says the special court has no jurisdiction over him, and is unlikely to appear in person for the appeal proceedings.
If the court upholds his sentence, he will serve his time in Senegal or in another member country of the African Union.
A cache of files from Habre's secret police recovered from an abandoned building in Ndjamena, Chad's capital, was used as evidence in the prosecution's case against the former president.
These documents alone are said to have revealed the names of 1,208 people who died in detention, and of almost 13,000 people who were victims of torture, extra-judicial execution, and arbitrary arrest.
Human Rights Watch researcher Reed Brody says the secret files provided a roadmap of how the repression of the Chadian people was carried out.
Who is Hissene Habre?
• Born in 1942 to ethnic Toubou herders in northern Chad
• Given scholarship to study political science in France
• First came to the world's attention in 1974 when his rebels captured three European hostages to ransom for money and arms
• Seized power in 1982 allegedly with the help of the CIA
• Ousted by current President Idriss Deby in 1990
• Accused of systematically persecuting groups he distrusted