The Yemeni authorities must clarify the whereabouts of journalist Muhammad al-Maqalih, who was seized by plain-clothed assailants in the capital Sana'a late on Thursday.
Human rights activists believe that he is being held by National Security officials (al-Amna Qawmi) in Sana'a.
The 49-year-old had recently been reporting on the ongoing conflict in the Sa'da region of northern Yemen, which has seen fierce clashes between rebels and government forces.
Muhammad al-Maqalih was snatched from Ta'iz street in Sana'a at 11pm local time on 17 September.
Amnesty International suspects his apparent disappearance is the result of his strong opposition and criticism of the government, in particular over the armed clashes raging in Sa'da and the killings of civilians by government forces.
Critics and opponents of the state in Yemen are often at risk of arrest and detention, particularly at times of political crises such as the current armed clashes in Sa'da.
Amnesty International has called on the authorities to clarify Muhammad al-Maqalih's whereabouts and to release him without delay if he is being held solely for his criticism of the government, which would make him a prisoner of conscience.
The Yemeni authorities should ensure that Muhammed al-Maqalih is protected from torture and other ill-treatment, and allowed prompt and regular access to lawyers of his choosing, his family and any medical treatment that he may require.
Yemen's Sa'da region, whose inhabitants are predominantly members of Yemen's Zaidi Sh'ia minority, has experienced several periods of conflict in recent years with recurrent armed clashes between government security forces and followers of the late Zaidi Shi'ia cleric, Hussein al-Houthi, who was killed in 2004.
The latest upsurge in violence began in mid-August, when the area was placed under a virtual state of emergency and government forces mounted an escalating series of attacks, including bombing raids against villagers and towns in an apparent attempt to crush the Houthi militants.
Last week, Amnesty International called on the Yemeni authorities to investigate all allegations of serious violations by their forces. These include the reported bombing raid on 16 September 2009 that is said to have killed about 80 civilians at Adi village in the Harf Sufyan area of Amran province, near Sa'da.