Alin Hilowle Hassan, director of Isha, a human rights organization in the Bay and Bakool region of southern Somalia, was captured at his home in Baidoa.
Members of human rights organization Social Action were stopped and detained earlier this month after leaving their office.
Artist Owen Maseko was arrested after participating in a show depicting atrocities that took place in the Matabeleland region in the 1980s.
An increase in harassment, arbitrary arrests, detentions, torture and other ill-treatment by security forces is anticipated ahead of April's elections.
Unrest across the country is linked to calls from supporters of President Gbagbo, for tens of thousands of "foreigners" to be excluded from the electoral roll.
Gertrude Hambira fled after five men and one women who identified themselves as officers from the Criminal Investigation Department raided her office.
Radio correspondent Ali Yusuf Adan was taken after the broadcast of a report alleging that the armed group had killed a man in the Wanleweyn district.