By Richard Galpin
BBC News, Moscow
One of Stalin's slogans was recently restored to a Moscow metro station
Russian President Dmitry Medvedev has made an outspoken attack on those seeking to rehabilitate former Soviet leader Joseph Stalin.
Millions of Soviet citizens died under Stalin's rule and Mr Medvedev said it was not possible to justify those who exterminated their own people.
He also warned against efforts to falsify history and defend repression.
Some Russian politicians have recently tried to portray Stalin in a more positive light.
Under President Medvedev's predecessor, current Prime Minister Vladimir Putin, Stalin was often promoted as an efficient leader who turned the Soviet Union into a superpower.
Mr Medvedev made the unusually critical comments in a videoblog posted on the Kremlin's website.
It appeared on the day the country is supposed to honour millions of people killed under Stalin's brutal regime which lasted from the late 1920s until his death in 1953.
President Medvedev said Stalin's mass killings could not be justified
Mr Medvedev said it was impossible to imagine the scale of repression under Stalin when whole groups of people were eliminated and even stripped of their right to be buried.
The president said there were now attempts to justify the repression of the past, and he warned against the falsification of history.
All this flies in the face of the current trend to promote Stalin as an effective manager and a leader who transformed the Soviet Union.
Under Mr Putin, the order was given for school history books to be re-written, highlighting Stalin's achievements.
In Moscow there is now even a Stalin-themed cafe and a metro station with one of Stalin's famous slogans on its walls. In northern Russia a historian investigating crimes committed by the former Soviet dictator was recently arrested.
It would appear there is a split within the Russian leadership on this highly sensitive issue.