Switzerland will try a former Gambian minister under ousted dictator Yahya Jammeh for crimes against humanity on Monday in a milestone case where a serial rape victim will testify after a multi-decade wait for justice.

Former interior minister Ousman Sonko will become the highest-ranking official to be tried in Europe under the principle of universal jurisdiction that allows grave crimes to be prosecuted anywhere, said Swiss campaign group Trial International which filed the complaint against him.

Nine Gambian plaintiffs will travel to Switzerland for the scheduled January 8 to 30 trial at the Federal Criminal Court in Bellinzona in a case rights activists see as ensuring global accountability for the worst atrocities.

Sonko, 54, faces charges including murder, multiple rapes and torture between 2000 and 2016 in what is Switzerland's second trial for crimes against humanity. He denies the charges.

"It has been a long period of waiting with anger and anxiety. But I am very optimistic and I feel so happy. I am smelling justice," said Madi Ceesay, a 67-year-old plaintiff who said he was detained and tortured under Sonko.

The defendant's lawyer Philippe Currat plans to ask the court to abandon the case, citing problems with the investigations and hearings.

"Since the beginning I have been stupefied by the way this file has been handled," he told Reuters, saying some evidence in the indictment was based on "secret" hearings in Gambia and interviewees had not been informed of their rights.

One of the plaintiffs is Binta Jamba who, according to the indictment, was raped many times by Sonko between 2000 and 2002 after he murdered her husband in connection with an alleged planned coup attempt.

In 2005 he held her captive for five days, beating her and raping her repeatedly, the indictment said. She fell pregnant by him twice and he paid for the abortions.

"Me and my family have been struggling with this for almost 25 years," she said in a message sent to Reuters.

"Without justice I will never have peace in my life."

Currat said he can prove Sonko was abroad during much of the period of the rape accusations.

He will also argue that many of the alleged crimes against humanity, including the rape charges, happened before a relevant Swiss law took effect in 2011 and are not admissible.

Sonko was arrested in early 2017 in Switzerland, where he was seeking asylum. Jammeh's 22-year repressive rule ended in January 2017 after he lost an election and was forced to flee.

Sonko could face a life sentence as a maximum possible penalty.

Currat said his treatment in Swiss jails has been cruel and he was denied food and given inadequate medical care.

Fatoumatta Sandeng, daughter of Solo Sandeng, a Gambian opposition activist killed in custody in 2016, said she is eager to look into Sonko's eyes in court.

She said: "If we don't hold people accountable, things like this will keep happening in Gambia, in Africa, all over the world."

Reuters