(Adds survivor's story)

By Oleg Shchedrov

BUTOVO, Russia, Oct 30 (Reuters) - Russian President Vladimir Putin paid his respects on Tuesday to millions of people killed under Soviet dictator Josef 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 were executed under Stalin and many more perished from abuse and disease in a vast network of prison camps, known as the Gulags.

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.

He said such tragedies "happen when ostensibly attractive but empty ideas are put above fundamental values, values of human life, of rights and freedom".

"Hundreds of thousands, millions of people were killed and sent to camps, shot and tortured," he said. "These were people with their own ideas which they were unafraid of speaking out about. They were the cream of the nation."

Historians estimate that between 20 million and 40 million died during Stalin's rule, tearing families apart and creating a climate of fear that haunted the Soviet Union.

A new history teaching manual partly authored by a top Putin strategist described Stalin as brutal, but also "the most successful leader of the USSR".

GREAT TERROR

Dozens of mainly older Russians laid flowers at a stone memorial outside the Moscow headquarters of the former KGB -- now the Federal Security Service -- to remember his victims.

Klavdia Suravykh came to see a small panel of photographs in memory of victims of Stalin's purges, and said she was the only survivor of a mass execution near Orlov, some 360 km (220 miles) south of Moscow, in 1941.

She said she lined up at the edge of a ditch with 300 other people. Gunfire killed her mother and everybody else, and the next line of victims fell on top of her.

"Everybody was killed there. Everybody. I was the only one..." she said, blinking back tears.

Stalin, who succeeded Vladimir Lenin, started a series of purges in the 1930s that became known as the Great Terror. The NKVD security service, the predecessor to the KGB, killed hundreds of thousands of people on trumped-up charges.

Butovo was just one of hundreds of killing grounds. More than 20,000 people are known to have been executed there between August 1937 and October 1938 alone. (Additional reporting by Chris Baldwin)

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