Tunisian Interior Minister Ali Laarayedh said authorities believe Belaid's death was planned and carried out by an extremist religious group, Tunisia Live reported.
Laarayedh denied any external involvement in the assassination.
Laarayedh said the suspects in custody range in age from 26-36 years old.
Earlier a source said a fundamentalist Islamist admitted involvement in Belaid's death saying a fatwa had been issued against the opposition leader.
Thousands of protesters demonstrated Saturday in Tunis, questioning the delay in investigating Belaid's killing.