He is a small, energetic looking man with thick spectacles and white hair. He speaks English with a thick, slightly American accent. As he stands in the queue outside Munich's criminal court in his beige zip-up jacket, he looks like any ordinary pensioner with time on his hands. He might have come to watch justice being done merely as an impartial observer.

Yet Thomas "Toivi" Blatt is no ordinary senior citizen. Aged 82, he is one of the last people alive to have survived the hell of the Nazi extermination camps in which millions were systematically slaughtered by brutal German and Ukrainian SS henchmen during the Second World War.

Only 82 people survived the death factory camps of Treblinka, Sobibor, Belzec and Auschwitz-Birkenau in Nazi-occupied Poland. Thomas Blatt is one of them. He still has a bullet lodged in his jaw to show for it. He got it when he was shot in the face and had to "play dead" after a miraculous escape from Sobibor in October 1943.

This Tuesday, having travelled from his home in California, he will enter the witness box in the clinical surroundings of Munich's modern neon-lit criminal court and recall the horror he endured as a 15-year-old Jewish prisoner in Sobibor, where 250,00 people including his parents and his 10-year-old brother were murdered.

Mr Blatt's testimony is expected to be crucial in deciding the outcome of what has been called the world's last Holocaust trial. The alleged former Nazi SS guard John Demjanjuk, a former Ukrainian citizen, who was extradited to Germany from Cleveland, Ohio last year, is accused of complicity in the murder of 27,900 mainly Dutch Jews who were gassed or beaten to death at Sobibor in the spring and summer of 1943.

Demjanjuk, who is 89 and suffering from numerous ailments, is refusing to speak at his trial, and follows the proceedings in court lying on a hospital stretcher with a baseball cap pulled over his eyes. He has denied complicity in mass murder.

Mr Blatt has not been able to identify Demjanjuk as having been one of the 100 Ukrainian guards at the camp, although he believes he was certainly there at the time he was a Sobibor prisoner. His testimony will nevertheless make it chillingly clear that anyone employed as a Sobibor guard was a vital cog in the machinery of genocide. "We were terrified of the Ukrainian guards at Sobibor. They were worse than the Germans – and I was there at the same time as Demjanjuk," he said.

Mr Blatt was born in Izbica, only 43 miles from the Sobibor death camp. A Nazi SS squad picked him up with his father, mother and young brother during a routine "Jew round-up" in April 1943. They were bundled into a truck. "The camp gate opened to reveal what looked like a beautiful village," he said. "We had heard what went on in there, but we refused to believe it. But I knew better, I was 15 and I realised that I was going to die," he added.

He remembers a man standing next to him peering through a hole in the truck's side at a group of Ukrainian guards equipped with bull whips. The man said, in Yiddish, how the place was "black" with them in reference to their black SS uniforms. Within a matter of seconds the Jewish prisoners were herded out of the truck and set upon by the screaming, whip-wielding guards who drove them up a path known as the "Himmelfahrtstrasse" or "road to heaven" towards the gas chambers.

"They mistreated us, they shot the old and the sick new arrivals who couldn't walk anymore. And they were the ones who drove the naked people into the gas chambers with their bayonets. They would come back with splashes of blood on their boots," Mr Blatt told The Independent on Sunday. "I often had to work a few feet away. If they refused to go on, they hit them and fired shots. I can still remember their shout of 'Idi siuda', which means 'come here'," he added.

Young Thomas saw his father being beaten with a club."Then I lost sight of him. I said to my mother, 'And yesterday I wasn't allowed to drink the rest of the milk, because you absolutely wanted to save some for today'. That strange remark of mine still haunts me today – it was the last thing I said to her. My brother stayed at my mother's side. They were all murdered in the gas chambers within the hour." A few weeks later, Thomas Blatt was forced to watch his best friend, Leon, being slowly beaten to death.

Middle-class Dutch Jews started being sent to Sobibor shortly after his arrival and the SS changed their tactics to trick them. "When a Dutch transport arrived, usually an SS man would hold a speech. He would apologise for the arduous journey, and said that for hygienic reasons, everyone needed to shower first. Then later they would be given jobs. Some of the new arrivals applauded. They had no idea what was in store," he recalled.

He survived only because the SS had executed a number of so-called "work Jews" at Sobibor the day before he arrived. The camp commandant was looking for replacements and 15-year-old Thomas pushed himself forward, pleading "Take me, take me!".

His jobs included polishing SS men's boots, sorting the clothes and shaving the hair off naked women prisoners before they were driven into gas chambers pumped full of exhaust fumes. It took up to 40 minutes for those inside to die. "We heard the whine of the generator that started the submarine engine which made the gas that killed them. I remember standing and listening to the muffled screams and knowing that men, women and children were dying in agony as I sorted their clothes. This is what I live with," he said.

He survived by supplying his Ukrainian tormentors with the gold coins he found in the victims' clothing after they were gassed. The guards used the money to pay for prostitutes. He tried to look healthy, strong and useful to avoid being murdered in the routine executions of the "work Jews". "I knew the Germans like it when you were clean and healthy looking," he said. "I watched out that my pants didn't get wrinkled and that they kept their creases and I always went around looking for possibilities to escape," he added. The chance came in October 1943 soon after a group of Jewish Red Army soldiers arrived at the camp. Within two weeks they had planned an uprising, which later became the subject of a book by Blatt The Forgotten Revolt, and an award-winning television film Escape from Sobibor. Thomas Blatt helped to kill 12 SS officers by tricking each one separately into believing that a fine leather coat had been saved for them from one of the Jewish transports. Prisoners armed with axes and knives killed the SS men as they arrived to collect their booty.

A breakout ensued. Jewish prisoners scaled the perimeter fence under a hail of gunfire from the camp watchtowers, which were still manned. The ones who got over the fence were blown up by mines that surrounded the camp. Mr Blatt escaped this fate because his jacket caught on the fence. He eventually got through the minefield by jumping through the pits in the ground caused by the explosions.

He and a friend bribed a Polish farmer to hide them under the floor of a barn in return for loot they took from the camp. But after stripping off some of their clothes and hiding them in a pit, the farmer decided it would be safer to kill them. The farmer shot at them and Thomas was hit in the face. He pretended to be dead and then managed to escape, spending the rest of the war hiding in woods or abandoned buildings and scavenging for food.

Mr Blatt says he is not seeking vengeance. "I don't care if he [Demjanjuk] goes to prison or not – the trial is what matters to me," he said.