The case has deepened concerns among human rights defenders about the recent suppression of street demonstrations intended to challenge strongman Prime Minister Hun Sen's nearly three-decade rule.
Rights groups say if convicted the 23 defendants -- most of whom have been detained for months without bail -- could face up to five years' imprisonment on charges including committing intentional violence.
Several hundred demonstrators gathered near the Phnom Penh Municipal Court as the trial began, holding a prayer at a police barricade amid cries of "Free the 23!"
At least four civilians were killed in early January when police opened fire on protesting textile factory workers who were calling for a minimum wage of $160 a month to make clothes for brands including Gap, Nike and H&M.
The International Trade Union Confederation has launched a campaign to "Free the 23", urging workers to lobby Cambodian embassies around the world.
AFP