Investigators digging at the site of a Soviet-run prison camp in the former East Germany have uncovered mass graves containing the bodies of 12,500 people, the Brandenburg state government said today.

The camp was at Sachsenhausen, north of Berlin, and was open from 1945 to 1950. Victims were said to have included real and supposed supporters of the defeated Third Reich, as well as citizens considered unfriendly to Communist authorities.

Until the Communist Government of East Germany collapsed in 1990, it was impossible to conduct research like that now under way at Sachsenhausen. Similar excavations are underway at other sites, and officials expect further discoveries like the one announced today.

The excavation around Sachsenhausen revealed 50 graves, each about 25 feet long and 13 feet wide. Under the earth, bodies were stacked in heaps as high as 15 feet and higher.

Pathologists have determined that most of the victims died of starvation, exposure or communicable diseases. Some had evidently been beaten. Most were children, adolescents and elderly people.

In the years after the end of World War II, occupying Soviet forces imprisoned thousands of Germans. Many were accused of war crimes, and their trials were perfunctory if they were held at all. Some were simply picked off the street, victims of Stalinist crackdowns.

The victims were taken to one of a network of prison camps. Some of them, like the one at Sachsenhausen and another at Buchenwald, were built on the sites of Nazi concentration camps.

The German Government estimated that 65,000 people died in those Soviet-run camps or in transportation to them.

During the four decades of Communist rule in East Germany, memorials were built at places like Sachsenhausen and Buchenwald. But the memorials implied that the camps closed at the war's end. They did not mention that in the post-Nazi era, the camps became brutal Soviet-run military prisons.

An official of the Brandenburg state government said a memorial to the victims of Stalinism would be erected at Sachsenhausen.

More than 200,000 inmates from various parts of Europe were imprisoned at Sachsenhausen from 1936 to 1945, when it was a "labor camp" run by the Nazis. Only about half are believed to have survived.