Huge cross marks Stalin purges
The wooden cross - 12.5m high (41 ft) and 7.6m wide (25 ft) - was placed in Butovo, at the site of a former execution ground.
At least 20,000 people were killed there by Stalin's secret police, the NKVD. The first killings occurred exactly 70 years ago.
Hundreds of people attended the ceremony south of the capital.
Events marking the 70th anniversary of Stalin's drive to purge opponents of his regime have been held throughout Russia.
The relatively small-scale ceremonies have been organised by religious or human rights groups rather than the government.
The BBC's James Rodgers in Butovo says the execution ground had previously been a firing range.
It did not seem necessary to change its name after 8 August, 1937, he adds.
Yulia Shcherbakova - now in her 70s - wanted to explain her personal tie to Stalin's terror.
"It's terrifying to think back. I remember people in our small house being arrested - people who lived below and above," she told the BBC.
"I was seven when my neighbour, a priest, was taken away - he disappeared without a trace. And everyone was afraid to mention his name."
The Siberian pine cross was erected as a centrepiece to a new memorial to Stalin's victims in Butovo.
Those executed there in 1937 and 1938 included about 1,000 priests, monks and nuns.
No-one knows precisely how many are buried at the site.
The cross was constructed at the Solovetsky Monastery in northern Russia, which was itself turned into a notorious prison camp during the purges.
The cross was delivered by boat, and part of its route followed the White Sea Canal, a Stalinist construction project which claimed the lives of thousands of convicts.
Seventy years after what is known as "the great purge", only a few thousand survivors remain.
Human rights groups say they have never been properly compensated, and most struggle to get by on a small state pension.