The World War II extermination of Europe's Jews by Nazi Germany began after the invasion of Poland in 1939 and increased in scale with the creation of death camps.

The Nazis called it the "Final Solution to the Jewish Question" and killed six million Jews from across Europe -- more than a third of the world's Jewish population at the time.

- Starvation, death squads -

The first massacres were perpetrated through starvation and mass shootings.

In Poland, Jews were imprisoned in ghettos from 1939 to late 1941, where many starved to death or died of disease.

The Nazis also dispatched mobile death squads called Einsatzgruppen which mowed down one million people in what is known as the "Holocaust by bullets", dumping them into mass graves.

They were mainly Jews and Soviet prisoners of war in Polish, Baltic and Soviet territories.

- Gas chambers -

The leader of the SS, Heinrich Himmler, and his deputy Reinhard Heydrich, introduced the gas chambers in 1941, an extermination technique tried out in Germany on disabled people.

At the Auschwitz-Birkenau camp near Krakow in southern Poland, the Nazis were already experimenting with Zyklon B, a cyanide-based pesticide. They used the chemical for the mass gassing of 600 Soviet prisoners and 250 Poles in September 1941.

"Operation Reinhard" led to the construction of three purpose-built extermination camps with gas chambers in occupied Poland.

Once the Belzec, Sobibor and Treblinka death camps were operational in 1942, the Nazis transferred people held in the ghettos to them, with about two million Polish Jews murdered.

- 'Final Solution' intensifies -

The intensification and coordination of the "Final Solution" was agreed at a landmark conference of government ministries and top-ranking Nazi and SS officials in the Berlin suburb of Wannsee on January 20, 1942.

The 15 participants at the Wannsee Conference, convened by Heydrich, agreed that 11 million Jews should be moved to death camps under the exclusive authority of the SS.

Jews from all over Europe were systematically deported from mid-1942 to six death camps: Auschwitz-Birkenau, Belzec, Chelmno, Majdanek, Sobibor and Treblinka.

At Auschwitz -- which became the symbol of the Holocaust -- more than 1.1 million people were killed, mainly Jews but also Roma, Soviet prisoners of war and Poles.

It also had work camps where German industry, notably the IG Farben chemical producer, used inmates as slaves.