The 89-year-old was deported in May from the United States to Munich, and has been in custody since then. He could face up to 15 years in prison if convicted for his alleged activities as a guard at the Sobibor camp in occupied Poland.
Demjanjuk has been deemed fit for trial, though his family says he suffers from a bone marrow disease and could have only months to live. In deference to his fragile health, his trial at the Munich state court has been limited to two 90-minute sessions per day.
Efraim Zuroff, the top Nazi hunter at the Simon Wiesenthal Center, said on his way into the court that it was important it was finally taking place.
"This sends a very powerful message that even if you didn't have the rank of an officer, you still have responsibility," Zuroff said.
Demjanjuk became a household name in the 1980s when he was extradited by the United States for trial in Israel on charges that he was the notoriously brutal guard at Treblinka who earned the moniker "Ivan the Terrible" for his deeds.
He was convicted in 1988 of war crimes and crimes against humanity, and spent seven years in prison until Israel's Supreme Court in 1993 overturned the conviction. It ruled that another person, not Demjanjuk, was actually "Ivan the Terrible."
Ukrainian-born Demjanjuk, a former Soviet Red Army soldier, is now accused of volunteering to serve as a guard under the SS after being taken prisoner by the Nazis in 1942.
According to the indictment, he served as a simple "wachmann," or guard, under the SS. As such, he is the lowest-ranking person to go on trial for Nazi war crimes.
The prosecution argues that, even with no living witnesses who can implicate Demjanjuk in specific acts of brutality or murder, just being a guard at a death camp means he was involved in the Nazis' machinery of destruction.
Before that, however, the prosecution must prove that Demjanjuk, who is being tried in Munich because he lived in the area briefly after the war, really did serve at the camp.
Demjanjuk maintains he was never at Sobibor and questions the authenticity of one of the main pieces of evidence - an SS identity card that prosecutors say features a photo of a young, round-faced Demjanjuk and that says he worked at the death camp.
He claims to be a victim of mistaken identity and says he was a Red Army draftee from Ukraine captured during the battle of Kerch in the Crimea in May 1942 and himself held prisoner until joining the so-called Vlasov Army of anti-communist Soviet POWs and others. That army was formed to fight with the Germans against the encroaching Soviets in the final months of the war.
Some of the most damning evidence comes from statements made by Ignat Danilchenko, a now-deceased Ukrainian who once served in the Soviet Army and was exiled to Siberia following World War II for helping the Nazis.
In 1979, he told the Soviet KGB that he served with the Ukrainian-born Demjanjuk at Sobibor and that Demjanjuk "like all guards in the camp, participated in the mass killing of Jews."
But there are inconsistencies in the Danilchenko statements, and the defense questions their validity.
Court sessions in the trial are scheduled through next May.
If convicted, Demjanjuk could be given credit in sentencing for some or all of the time he spent behind bars in Israel. Even if he is acquitted, however, Demjanjuk likely will have to remain in Germany as he has been stripped of his U.S. citizenship.