The main purpose of the visit (his fifth) was to assess how well parliament functions in upholding the rights of ordinary Cambodians.
The rights group Adhoc has so far recorded at least 124 arrests in land disputes this year, with at least 18 people now in detention.
Sam Rainsy Party officials said Friday they believe the true killer or killers remain at large.
The government estimates some 730 juvenile prisoners—those aged between 14 and 17—are incarcerated in the country’s 25 prisons.
The major increase in counter-narcotics has come this year, with 475 alleged drug offenders already arrested.
The petition drives comes as the UN’s special human rights envoy for Cambodia, Surya Subedi, arrives in Phnom Penh on Monday.
Subedi is also expected to speak directly with residents in the Beoung Kak lake dispute.
Cambodia has been influenced largely by the French and the Chinese in its modern history, with China’s influence growing today.
Rights officials said that the order was understandable during ongoing operations, but that it could be abused.
Moeung Sonn, who has fled the country, is facing a two-year prison sentence following a suit brought by the government in 2009.
About 40 Cambodian police and military police received FBI training in interview techniques this week.
Chea Kim, the wife of Ho Sok, says 29 government officials and six judges worked together to sell 13 hectares of land in 1998.