Yemeni anti-war activist and political analyst Hisham Al-Omeisy was pulled off the streets of the Yemeni capital Sanaa by 15 armed security officers on Monday, August 14.
Amnesty International says the 38-year-old father of two young sons has been detained since then by Yemen's Houthi-Saleh forces without access to a lawyer or his family.
The NGO is calling on the Houthi's National Security Bureau (NSB) to immediately and unconditionally release the leading political activist, saying that four days after his arrest, the NSB are still holding Mr Al-Omeisy incommunicado in an undisclosed location.
"Hisham Al-Omeisy has been detained without charge or a court appearance in breach of Yemen's constitution, which requires anybody arrested to be presented in court within 24 hours," Amnesty International Middle-East campaign director Samah Hadid said.
"This detention illustrates the lengths to which local Houthi-Saleh authorities are willing to go to silence peaceful activists.
"Hisham Al-Omeisy is a prisoner of conscience, whose only 'crime' is peacefully exercising his right to freedom of expression, and he must be released immediately."
Mr Al-Omeisy is a political activist who rose to fame during the Arab Spring and the ongoing Yemeni conflict.
He is a one of the most high-profile activists in Yemen, using social media to show the world the horror of living under daily Saudi-led airstrikes on the north of the country.
Mr Al-Omeisy is a regular commentator on the ABC and last year he featured in the Foreign Correspondent documentary Yemen: The War on Children, bravely calling out all sides of the fighting in Yemen for their abuses of human rights.
Since mid-2016, Yemen has witnessed a surge in arbitrary arrests, detentions and enforced disappearances by the various warring parties in the country.
Those targeted include critics and political opponents, journalists, human rights defenders and minorities, such as members of the Baha'i community.