A 93-year-old former Auschwitz guard begged for forgiveness as he took the stand during the first day of his trial in Germany, where he is accused of 300,000 counts of accessory to murder.

Oskar Groening faced a courtroom packed with survivors of the Holocaust and their relatives in the town of Lueneburg, south of Hamburg, on Tuesday.

Groening testified that he volunteered to join the SS in 1940 after training as a banker and helped collect and record money and belongings stolen from Holocaust victims between May and June 1944.

Some 425,000 Jews from Hungary were brought there during this period and at least 300,000 gassed to death. His job earned him the name "Accountant of Auschwitz".

"For me there's no question that I share moral guilt ... I ask for forgiveness," he was quoted as saying by Sky News to the panel of judges hearing the case during an hour-long statement.

Auschwitz survivor Eva Pusztai-Fahidi, 2nd from right, from Budapest and her 23-years-old granddaughter Luca Hartai, right, leave the court hall during the noon breaks of the trial against former SS guard Oskar Groening (AP)

"I share morally in the guilt but whether I am guilty under criminal law, you will have to decide," he said.

Groening described some of the murders that he witnessed at Auschwitz. On his first day on the ramp where Jewish prisoners exited the trains, he saw an SS colleague grab a crying baby and slam its head against a truck until it was quiet.

"I was so shaken. I don't find what he did good at all," Groening said, adding that he later requested a transfer from out of Auschwitz.

He also described an incident in 1942 when he witnessed naked Jews being forced into a converted farm house near the camp. He said a fellow officer locked the door, put on a gas mask, opened a can and poured its contents down a hatch.

"The screams became louder and more desperate but after a short time they became quieter again," Groening said.

Prosecutors say that by serving as a guard and helping the Nazi regime benefit economically, he is complicit in the atrocities committed at Auschwitz. Groening has acknowledged that he knew of the killings but claims he committed no crime by not directly taking part in them.

He could spend the rest of his life in prison if found guilty of the charges against him.