Thousands of people held candles and prayed at a ceremony unveiling a stark gray boulder placed near the KGB's Lubyanka headquarters as a monument to the millions once imprisoned in Soviet labor camps.

The Memorial Society, dedicated to those who suffered during the reign of terror initiated by Josef Stalin, staged the emotionally charged demonstration Tuesday in downtown Moscow in the shadow of the looming black statue of Felix Dzherzhynsky, founder of the secret police.A ceremony was also held in Leningrad.

"The best minds of our country were exterminated in the prison camps," said Genrikh Altunyan, an Armenian deputy from the Ukraine. "We must do everything possible so that this will never happen again."

Altunyan told the crowd, which police said numbered about 5,000 people, that he served time in a labor camp several times in the 1970s and 1980s.

Throughout the rally, people passed up red carnations to be placed on the boulder's gray marble base with the inscription: "This rock from the grounds of the Solovetsky Labor Camp has special significance and was erected by the Memorial Society in honor of the millions of victims of the totalitarian regime."

Demonstrators, bundled up against a brisk autumn evening, lighted candles, held up photographs of loved ones who disappeared in the labor camps, observed a minute of silence and sang in a prayer service led by Russian Orthodox priest Gleb Yakushin.

Several demonstrators interviewed in a Soviet television news report on the ceremony, broadcast later Tuesday night, broke down in tears as they told of relatives who had died in prison camps.

Nearly all the banners and posters held aloft at the demonstration carried anti-KGB or anti-Communist slogans.

Speakers demanded that the KGB turn over its archives and allow the establishment of an institute to study the infernal machine that condemned millions to work in the Gulag camps on trumped-up charges often based on forced denunciations by acquaintances.