"We will start the with the files that contain information of public and historical interest," said Ivan Komitski, the head of the Interior Ministry's archives department.
Some 253,000 files have already been declassified, and experts continue work on more than 2 million archived secret files, a ministry statement said. Most of them are related to the work of the former communist police force until 1989, when Bulgaria's communist regime collapsed.
Journalist and researcher Hristo Hristov, who has investigated several Cold War cases, including the notorious 1978 killing of Bulgarian dissident Georgi Markov in London, said this should have been done three years ago.
"A 2002 law on the protection of classified information clearly stipulated that the former communist services' files are not secret any more and should be transferred to the State Archive within a year," Hristov said.
"The Interior Ministry is doing this three years late, and instead of placing the whole array of files in the State Archive, it starts a partial declassification, without even announcing what has been declassified and what remains secret," he said.
Officials said this was a matter of procedure, the AP reports.
Hristov also argued that the legal declassification requirement referred to the archives of all former communist services, including the files now kept by the National Intelligence Service, which are still secret.
The United States and NATO are conducting provocative activities both in airspace and waters of the Black Sea, Russian Defense Minister Sergei Shoygu said