Germany will inaugurate the first national memorial to the estimated 300,000 ill and disabled people systematically murdered by the Nazis on Tuesday, at a ceremony with victims’ relatives.

The site next to the Tiergarten park is the fourth and likely final major memorial in Berlin’s city center to groups targeted in the Holocaust, following monuments dedicated over the last decade to Jewish, gay and Roma victims.

It honors a long-neglected class of people with few vocal advocates in a move historians say is long overdue.

Uwe Neumaerker, director of the Holocaust memorial foundation in Berlin, said the slaughter of patients and residents of care homes marked “the first systematic mass crime of the National Socialist regime.”

“It is considered a forerunner of the extermination of European Jews,” he said.

The German parliament voted in November 2011 to erect a memorial to the victims of the Nazis’ cynically labelled “euthanasia” program adjacent to Berlin’s renowned Philharmonie concert hall.

It includes a wall-like sculpture made of blue glass and information panels detailing the Nazis’ campaign to exterminate the sick, the physically and mentally handicapped, those with learning disabilities, and people branded social “misfits.”

The memorial is on the site of a now-demolished elegant villa at Tiergartenstrasse 4 where more than 60 Nazi bureaucrats and like-minded doctors worked in secret under the “T4″ program to organize the mass murder of sanatorium and psychiatric hospital patients deemed unworthy to live.

“We finally have a worthy place to keep alive the memory of the some 300,000 disabled and ill people who were murdered,” a deputy speaker of the German parliament, Ulla Schmidt, said.

The German culture minister, Monika Gruetters, and Berlin Mayor Klaus Wowereit will speak at the opening of the memorial, along with two relatives of T4 victims.

One, Hartmut Traub, will pay tribute to his uncle Benjamin, a schizophrenic who was murdered in a gas chamber in 1941 at the age of 27.

Between January 1940 and August 1941, doctors systematically gassed more than 70,000 people at six sites across the German empire.

From August 1941 until the war’s end in 1945, tens of thousands more died through forced starvation, neglect or fatal doses of painkillers such as morphine administered by purported caregivers.

Many underwent bizarre medical “experiments” and forced sterilizations.

Few of the killers were brought to justice after the war, despite high-profile trials like those of doctors at Nuremberg 1946-47, and many of the implicated medical professionals simply continued with their careers.

Neither West Germany nor the communist East recognized or compensated the survivors, and many families harbored a sense of shame about relatives who were seen as less than human.