Harry Smith, Al Jazeera's correspondent in Munich, said Demnjanjuk would address the court in his native Ukranian, although proceedings will be carried out in German.
"He is being tried here in Munich because he once spent some time here at a displaced persons camp before emigrating to the United States."
Demjanjuk says he fought in the Red Army before being captured by the Germans in 1942, and said that while he was recruited as a camp guard, he was not at Sobibor.
But Israeli and US courts have established he was at the camp, where prosecutors say Jews were killed with a toxic mix of carbon monoxide and carbon dioxide.
"When the bell tolls for John Demjanjuk, it is also tolling for every other war criminal."
Rabbi Marvin Hier
Ulrich Busch, Demjanjuk's lawyer, said that even if it could be proved his client was at Sobibor, he would have been there under duress and could not now be held responsible for the atrocities carried out at the camp.
But the prosecution is expected to argue that although there are no living witnesses who can implicate Demjanjuk in specific acts of brutality or murder, being a guard at a death camp means he was involved in the Nazis' machinery of destruction.
More than 30 co-plaintiffs are expected at the trial, many of whom lost family members at Sobibor.
Jewish groups and victims' families say that the case is symbolic.
"We should not make the mistake of thinking that a case against one war criminal is a case against just one man," Rabbi Marvin Hier, Dean of the Wiesenthal Center in Los Angeles, said.
"When the bell tolls for John Demjanjuk, it is also tolling for every other war criminal. Even if it just gives them sleepless nights," he told the Reuters news agency.
Demjanjuk was sentenced to death in Israel in 1988 for being "Ivan the Terrible" - a particularly sadistic Nazi guard, but after five years on death row it emerged the court had convicted the wrong man.
The Ukranian emigrated to the US in 1951, becoming a naturalised citizen seven years later, and worked in the auto industry. He was extradited in May this year.