The court rejected a defense motion to suspend the trial, upholding the confinement of a man who once topped the Simon Wiesenthal Center's list of most-wanted war criminals.
German state prosecutors accuse Demjanjuk of assisting in killings at the Sobibor death camp in Poland, where prosecutors say at least 250,000 Jews were killed.
Early in the day, the 89-year-old sat silently in his wheelchair and showed no emotion as 12 plaintiffs, some of them with tears in their eyes and their voices breaking, described how they had survived the Holocaust while their relatives perished.
After a recess, officials set up a bed in the courtroom for Demjanjuk, after a doctor said he had complained of back pain.
A prison official quoted in Tuesday's edition of the Tageszeitung newspaper said Demjanjuk regularly took breaks in the prison yard, either in his wheelchair or with the assistance of a walker. He also had been reading Ukrainian newspapers and preparing meals of salad, the official, Michael Stumpf, added.
Demjanjuk denies he was involved in the Holocaust and his family insists he is too frail to be standing trial.
One of the plaintiffs, 87-year-old Philip Jacobs had to be helped onto the witness stand, where he said he felt guilty to have survived the Holocaust while his parents and fiancee perished.
"Sobibor is an unhealed wound," the former pharmacist said.
Another plaintiff, Robert Cohen, 83, whose brother and parents were killed at Sobibor, described his experiences at Nazi death camps including Auschwitz.
"We didn't know what was going on," the Amsterdam-born pensioner told the court during the morning session. "We thought we had to work."
Due to Demjanjuk's frailty, hearings are limited to two 90-minute sessions a day. His case is likely to be Germany's last major Nazi-era war crimes trial.
Demjanjuk was born in Ukraine and fought in the Soviet army before being captured by the Nazis and recruited as a camp guard. He emigrated to the United States in 1951.
In May, he was extradited from the United States where he had lived in a suburb of Cleveland, Ohio.
Demjanjuk has acknowledged being at other camps [NOT TRUE!] but not at Sobibor, which prosecutors say was run by between 20 and 30 members of the Nazi SS and up to 150 Soviet former prisoners of war.
In the Sobibor gas chambers, Jews died within 20 to 30 minutes after inhaling a toxic mixture of carbon monoxide and carbon dioxide, prosecutors said. Groups of about 80 were forced into gas chambers measuring about 4 by 4 meters (13 ft by 13 ft).
The trial is scheduled to continue on Tuesday [22Dec2009].
(Reporting by Christian Kraemer; writing by Paul Carrel and Brian Rohan; Editing by Jon Hemming)