The 21 detainees were among 23 arrested in deadly labor protest crackdowns in January.
21 defendants charged with intentional violence and destruction of property during garment factory workers strike in Phnom Penh on January 3.
A confederation of 16 labor unions says it will hold another mass protest if the government refuses to release 21 detained labor activists.
The leaders from 16 unions met Monday to decide what to do after two of 23 workers were released from custody on Friday.
Riot police and security forces barred an opposition delegation from meeting with 23 protest detainees in Kampong Cham province.
Families of garment workers shot dead on Phnom Penh streets early last month have appealed for justice to be done for their loved ones. Colin Lovett narrates this report by Say Mony from Phnom Penh.
The crackdowns have been widely condemned, but no perpetrators have been arrested or reprimanded.
Representatives of nine unions met together have made seven resolutions to seek better conditions and wages for workers in Cambodia’s factories.
Two women wounded in the 2011 protest shooting in Svay Rieng province have seen their conditions worsen.
At least four people were killed when police fired on demonstrators during the Jan. 3 crackdown, no one has been arrested or reprimanded in the wake of the shootings.
The protesters gathered in New York despite a cold snap that dropped temperatures and snow on the city.