A Swiss court on Wednesday convicted a former government minister from Gambia of crimes against humanity under ex-dictator Yahya Jammeh and jailed him for 20 years, in a historic verdict using universal jurisdiction in Europe.

The Federal Criminal Court found Ousman Sonko guilty on several counts of intentional homicide, torture and false imprisonment. Sonko, who was dismissed as Gambia's interior minister in 2016, was acquitted of rape.

"The trial chamber found Ousman Sonko guilty of multiple counts of intentional homicide, multiple counts of torture and multiple counts of deprivation of liberty, each as a crime against humanity," the court said in a statement.

"The trial chamber concludes that Ousman Sonko committed these crimes.... as part of a systematic attack against the civilian population."

The judgment can be appealed.

Sonko is the highest-ranking official ever to be tried by a European country using universal jurisdiction which allows the most serious crimes to be prosecuted anywhere.

Geneva-based campaign group TRIAL International filed the original complaint against Sonko and supported the plaintiffs.

Sonko denied the charges and accused the plaintiffs of lying while denouncing his seven-year pre-trial detention - a period he said was partly spent in solitary confinement.

Sonko fell out with Jammeh in the final months of Jammeh's 22-year repressive rule, which ended when the leader was forced to flee to exile in Equatorial Guinea in January 2017 after an election defeat. That same month, Sonko was arrested in Switzerland while seeking asylum.

Switzerland's public prosecutor had sought the maximum penalty of life imprisonment.

"The conviction of Ousman Sonko, one of the pillars of Yahya Jammeh's brutal regime, is a major step on the long road to justice for Jammeh's victims," Reed Brody, a war crimes prosecutor attending the trial, told Reuters.

"The long arm of the law is catching up with Yahya Jammeh's accomplices all around the world, and hopefully will soon catch up with Jammeh himself," he said.

The case is Switzerland's second ever civilian trial for crimes against humanity and the Bellinzona court heard details between January and March of crimes allegedly committed between 2000-2016 either by Sonko or on his watch.

Human rights groups in Gambia hailed the verdict.

Demba Ali Jawo, Chairman of the National Centre for Victims of Human Rights Violations, said he hoped it would serve as a deterrent to "recalcitrant serving and future public officials that justice would be served," regardless of how long it took.

"However, some people wish that (Sonko) would be repatriated to The Gambia to serve his sentence in the same harsh prison conditions that their victims had endured here," he said.