The documents, dating from 1920 to 1950, are expected to shed new light on some of the most notorious excesses of the post-revolutionary and Stalinist eras, including Stalin's forced collectivisation of agriculture in the early 1930s, in which up to 10 million people died.

The archives, which include some two million documents, also cover the political purges of the late 1930s, which saw hundreds of thousands of party members executed as counter-revolutionaries or shipped off to gulags.

Historians and human rights activists reacted cautiously to the news, pointing out that only relatives of those "purged" would be able to study the documents.

Arseny Roginsky, a spokesman for the Russian human rights organisation Memorial, told the Guardian: "We welcome this but it's not sensational. This process of opening up the archives started under Boris Yeltsin in 1992. The problem is that it is up to the federal security service [the FSB, successor to the KGB] to decide what gets released and what stays secret. Most of these documents concern people subsequently rehabilitated.

"There is no independent historical committee allowed to evaluate this material. The Soviet Union's wartime relations with other states are completely off limits. And the biggest problem is that there is no catalogue: finding stuff is very difficult."

The FSB confirmed today that the archives would be made available only to relatives of those who were "purged".

Vasily Khristoforov, head of the FSB's archives and registers department, said family members could apply. He said the documents would be made available in a public reading room and that 1,500 requests were approved last year, under a policy of liberalising access to previously secret documents.

Most government archives were classified as state secrets during the Soviet era. A 1992 presidential decree declassified materials on Soviet-era repression. But Lyudmila Alexeyeva, a veteran human rights advocate, told Associated Press that access was again restricted in the late 1990s without any explanation.

She said the FSB may be lifting recent curbs to improve its image but lamented: "A small window is being opened again but 87 years later we are finally opening it up but only to relatives."