The Asian Human Rights Commission has asked King Norodom Sihamoni to push for more independence in Cambodia's notoriously corrupt courts.
In a letter to the king, the Hong-Kong based organization said the king could move the country towards democracy and the rule of law.
The new king typically keeps political affairs at arm's length, but the push comes as he prepares for a meeting with a judicial council to decide nationwide rotations of judges.
Cambodia continues to suffer from rights violations, land grabs, strike-breaking and suppression of the press, the group said, in a letter dated Dec. 10.
Ministry of Interior spokesman Khieu Sopheak denied the claims, saying Cambodian authorities followed the law.
"This year there has not been anyone arrested, detained or sent to prison because of expressing his or her opinion, because of assembly," he said. "We did not do that at all."
A Royal Palace official said he had not received the letter yet, and confirmed King Sihamoni would return to Cambodia on Wednesday.