Germany is marking the 75th anniversary of a famous plot to kill Adolf Hitler, honouring those who resisted the Nazis as pillars of the country's modern democracy.

Key points: An estimated 200 to 300 people were involved in trying to kill Hitler in July 1944

An estimated 200 to 300 people were involved in trying to kill Hitler in July 1944 Co-conspirators in the plot were long stigmatised as traitors

Co-conspirators in the plot were long stigmatised as traitors German Chancellor Angela Merkel said the resistance is an "example" to all

Chancellor Angela Merkel paid tribute ahead of the anniversary to executed plot leader Colonel Claus von Stauffenberg and his fellow conspirators.

"Only if we understand our past can we build a good future," she said.

Colonel von Stauffenberg tried to kill Hitler with a briefcase bomb on July 20, 1944, during a meeting at his headquarters in East Prussia.

Hitler escaped the full force of the blast when someone moved the briefcase next to a table leg, deflecting much of the explosion.

The plot crumbled when news spread that Hitler had survived. Von Stauffenberg and his fellow conspirators were executed within hours.

Colonel von Stauffenberg was one of the leaders of the failed attempt to assassinate Hitler. ( AP: Markus Schreiber )

Plotters initially shamed and forgotten

The story had little resonance in the immediate post-World War II years, when many still viewed the July 20 plotters as traitors, as they had been painted by the Nazis in the aftermath of the failed assassination.

The resistance against the Nazis only came to be "labouriously accepted" over subsequent decades, said Johannes Tuchel, director of the German Resistance Memorial Centre, and even in the 1980s many believed its memory would fade away.

Only in 2004 did a survey show that a majority of Germans believed the resistance to the Nazis was "important for [their] political culture," he added.

"Those who acted on July 20 are an example to us," Ms Merkel said last week in her weekly video message.

"They showed that they followed their conscience and set their stamp on a part of German history that otherwise was defined by the darkness of Nazism."

Mr Tuchel said von Stauffenberg is a "symbolic figure" of the resistance, an officer who evolved from supporting Nazi policies to becoming a ferocious opponent of the regime after Hitler's invasion of the Soviet Union in 1941.

Angela Merkel said understanding the past was important for the future. ( AP: Michael Sohn )

He acknowledged that the resistance within the German military was, in overall terms, tiny: 200 to 300 people were involved in the July 20 plot.

The German military had some eight million men under arms at the time, and only "a handful or two" of its more than 1,000 generals and admirals participated.

But the memorial Mr Tuchel heads, in the Berlin complex where von Stauffenberg worked and was executed, seeks to display the full breadth of German resistance to Hitler's regime after the Nazis took power in 1933.

Students in Munich formed the White Rose movement, distributing pamphlets urging "passive resistance" starting in 1942.

Its leaders included siblings Hans and Sophie Scholl, who were executed in 1943 and who have also become resistance icons.

Helmuth James von Moltke's so-called Kreisau Circle started working in secret to end the dictatorship in 1940.

Co-conspirators are being remembered in an exhibition in Berlin. ( AP: Markus Schreiber )

And in 1938, carpenter Georg Elser attempted to kill Hitler and other senior Nazi leaders at an event in Munich, but was thwarted as the Nazi leader unexpectedly left the room minutes before a bomb exploded.

Mr Tuchel conceded that, even now, there are shortcomings in historians' knowledge of the resistance and promised more research in the coming years into the role of women who opposed the Nazi dictatorship, responding to a recent call from Parliament.

Merkel urges action against far-right extremism

This year's anniversary comes weeks after the killing of a regional official from Ms Merkel's party who had supported the Chancellor's welcoming approach to migrants.

A man with previous convictions for violent anti-migrant crime was arrested as the suspected killer.

"Today, we are obliged to confront all tendencies that want to destroy democracy — including right-wing extremism," Ms Merkel said in her message.

AP