Thousands of Cambodians gathered for a Buddhist ceremony at the Choeung Ek “killing fields” Tuesday, in a day of remembarence.

As many as 20,000 were dumped in the mass graves outside in Phnom Penh under the Khmer Rouge. They were memorialized by a giant glass stupa full of skulls, where Cambodians made offerings Tuesday.

Phnom Penh Governor Kep Chuktema said the day was necessary to ensure atrocities did not occur in the future.

“I cannot forget the brutality of the Khmer Rouge,” said Ou Savorn, whose parents, brothers and sisters were killed by the regime. “I pray for the killing fields not to return to Cambodia.”