Among them was a Burmese woman who spent seven years in prison, as well as Cambodian land activist Tep Vanny, an outspoken community leader who has also been jailed.
Vann Sophath says he was threatened by security guards of the Khun Sea company, which belongs to in-laws of the prime minister.
About 50 Cambodian youths were prevented by city security personnel from performing a peace march in the capital on Tuesday.
The villagers say a Vietnamese rubber company began developing in the area in 2008, ultimately evicting some 400 families from their homes across more than 2,000 hectares of land.
Outside the court Friday, supporters for the 21 detainees gathered, condemning the court’s decision.
People can use social media, such as Facebook, in Vietnam, but some have been arrested for things they’ve posted, experts say.
The new study found that 770,000 people, or 6 percent of the population, have in some way been affected by land disputes since 2000.
Human rights workers on Wednesday condemned the court’s treatment of 23 labor activists, who were detained in January and are now seeing continued delays in their trials.
Phnom Penh Municipal Court on Tuesday continued a trial against 23 labor activists who were arrested during demonstrations in May.
The tribes say some 2,000 families could be affected by a concession granted to the company, which has begun clearing some 47,000 hectares of land in the province.
An estimated 1,000 riot police were deployed to main streets surrounding the courthouse, as around 100 supporters gathered to call for the release of all 23.
Flavia Pansieri, UN Deputy High Commissioner for Human Rights, says freedom in Cambodia has worsened.