The Australian-based Refugee Action Coalition says up to 150 refugees, including at least one family group with children, have now been arrested on Nauru.
The Coalition's Ian Rintoul says a 13 year-old has been arrested as a protest leader.
He says at least 40 men are in the Yaren police station while female refugees are being held in a separate location, though two have been taken to hospital after collapsing.
The arrests come as the Nauru government has imposed heavy-handed limits on what Mr Rintoul calls a campaign of non-co-operation by the refugees.
A notice distributed on Tuesday says groups of three or more which do not disperse within 15 minutes could be arrested and face charges carrying penalties of up to three years in jail.
The refugees staged protests last Friday and on Sunday.
The Nauru government has accused the refugees of violence but Mr Rintoul says the only violence has come from the Nauru police or Nauruan civilians.
Attempts to reach the Nauru police or the government for clarification have been unsuccessful.