Russian President Vladimir Putin paid his respects on Tuesday to millions of people killed under Soviet dictator Joseph Stalin and called for the country to unite to prevent a repeat of its tragic past.

Putin, a former KGB spy, marked Russia's annual day of remembrance for the victims of Stalin's purges with a visit to Butovo, a military training ground near Moscow where tens of thousands of people were executed by firing squads.

Millions of people were executed under Stalin and many more perished from abuse and disease in a vast network of prison camps, known as the Gulags.

The victims included priests and royalists but also huge numbers of people who were simply caught up in an indiscriminate spiral of killing. This year Russia marks the 70th anniversary of the bloodiest period of the purges.

Putin attended a memorial service with Patriarch Alexiy II, head of the Russian Orthodox Church, after passing a field criss-crossed with mass graves.

"We know very well that 1937 was the peak of the purges but this year was well prepared by years of cruelty," Putin said beside a mass grave after laying flowers at a memorial.