Dozens of Khmer Rouge victims held a sombre vigil at Cambodia's notorious torture jail Thursday on the eve of the final verdict in the trial of the prison chief who oversaw the deaths of thousands.

Around 100 survivors and relatives held a tearful minute-long silence in the courtyard of S-21 prison, now a genocide museum commemorating the 15,000 people who were tortured and killed there in the late 1970s.

Many expressed hope that Cambodia's UN-backed war crimes court will hand down a harsher sentence on appeal for prison commandant Kaing Guek Eav, better known as Duch, who was sentenced to 30 years in 2010 for his part in the killings.

"We hope there will be justice," said Bou Meng, 71, one of the few people to make it out of S-21 alive.

"We have all been waiting for this for so many years," he told AFP, adding that Friday would be "a historic day closing the dark chapter".

Duch was the first Khmer Rouge cadre to face the international tribunal, set up seek justice for victims of the 1975-1979 regime blamed for the deaths of up to two million people through starvation, overwork and execution in a bid to create an agrarian utopia.

During his trial Duch repeatedly apologised for his role at S-21, but surprised the court in his closing statement by asking to be acquitted. Observers say that's unlikely to happen.

"Life imprisonment or 45 years in jail, that's what I will accept," said Bou Meng, echoing the sentence demanded by the prosecution on appeal. "If it's different from that... I'll be very disappointed."

If the original verdict is upheld, Duch, 69, could be freed in under 18 years because of time already served.