Amnesty International is urging the public to call for the release, or guarantees of safety, of 42 people being held at Manokwari Police station in the north-west of the Indonesian province of Papua.
Those arrested were detained after commemorations to mark the 41st anniversary of the declaration of Papuan Independence in 1961.
Amnesty says no-one has been allowed to talk or see those being held and there are fears that they are vulnerable to ill treatment.
The human rights organisation has urged the Indonesian authorites to guarantee that those arrested will not be tortured or ill treated.
They also want the detainees to be given immediate access to lawyers and medical treatment.