Former refugees at the congress said they wanted to share in the freedoms they were given.
The Diamond Bridge stampede was the worst disaster in Cambodia’s modern history, leaving 354 dead and 393 seriously injured.
Cambodia exports labor to Japan, Malaysia, South Korea and Thailand especially.
Two years ago, the Boeung Kak lake area in central Phnom Penh was home to 4,000 families.
Land disputes continue to be an ongoing problem for many Cambodians, who settled on land after the Khmer Rouge era.
The union was planning to demonstrate against rising prices of food, fuel and other living expenses.
The 58-article law was sent to the Council of Ministers for approval last week, after which it will pass to the National Assembly.
The July meeting in Chicago was to set a strategic plan for the organization for the next two to three years.
The government says it needs the NGO law to better manage the thousands of organizations operating in the country.
The three men, who deny assaulting the police, say they are afraid the summonses will mean their arrest.
Only three-quarters of those surveyed in 2010 had heard of parliament, but of those, only two-thirds knew what it does.
About 250 families are laying claim to some 800 hectares of land they say was taken by a private company in 2005.