John Demjanjuk, accused of helping to murder more than 27,000 Jews at a Nazi death camp, has gone on trial in the German city of Munich, BBC reported.
Mr Demjanjuk, who is 89 and was deported from the US in May, entered the courtroom in a wheelchair. His eyes were closed but he seemed conscious.
He denies being a camp guard at Sobibor, in Nazi-occupied Poland.
The trial is expected to last until May and, if found guilty, Mr Demjanjuk could be sentenced to 15 years in jail.
The trial's first session was delayed, AFP news agency said, as large numbers of people tried to gain access.
Witnesses have been arriving for the court session, but organisers were overwhelmed by the crowds of people trying to get in, including journalists and relatives of Holocaust survivors.
Thomas Blatt, a Sobibor survivor, told journalists on his way into court that he was not looking for revenge.
"I'm here to tell the way it was years ago, I don't know Demjanjuk in person," he said.
John Demjanjuk is a retired US car worker, but he stands accused of having helped the Nazi death factory to function.
Prosecutors say that, as a camp guard at Sobibor, he pushed thousands of Jewish men, women and children to their death in the gas chambers.
Mr Demjanjuk was born in Ukraine and captured by the Nazis while fighting in the Soviet army. He denies even being at Sobibor.
Over 60 years later, this may be Germany's last big war crimes tria
But the BBC's Oana Lungescu in Munich says that, as the first to focus on a low-ranking foreigner rather than a senior Nazi commander, it breaks new legal ground.
As Mr Demjanjuk is 89 and in poor health, doctors have asked that hearings should be limited to two 90-minute sessions per day.
There are no living witnesses in this case, but over 30 people listed as joint plaintiffs are expected to testify about what happened at Sobibor, described by investigators as hell on earth.
Two are camp survivors, others lost relatives or their entire families among the 250,000 people murdered there.
This is the second time John Demjanjuk is appearing in court.
Two decades ago, he was sentenced to death in Israel, convicted of being Ivan the Terrible, a notoriously sadistic guard at the Treblinka death camp.
But that ruling was overturned after new evidence showed that another Ukrainian was probably responsible.
John Demjanjuk Nazi crimes trial starts in Munich
John Demjanjuk, accused of helping to murder more than 27,000 Jews at a Nazi death camp, has gone on trial in the German city of Munich.
