Yemeni authorities arrested the head of a Shiite opposition party, Hassan Zaid, at Sanaa airport on Tuesday as he was headed for the Saudi city of Jeddah, his son said.
"My father was detained," Mohammed Hassan Zaid told AFP. "He was travelling to Jeddah when he was detained and not allowed to leave."
Zaid said the whereabouts of his father, leader of the party Al-Haq, remained unknown although "everybody knows the reason is political."
He pointed the finger at "the national security and those behind it," apparently referring to relatives of President Ali Abdullah Saleh, who was flown to hospital in Riyadh last month with wounds sustained in a bomb blast.
Saleh has not appeared in public since the attack on his Sanaa palace compound, raising uncertainty over his return to power following anti-regime protests which have gripped Yemen since late January.
However, members of his family retain a firm grip on the impoverished state's security services.
Al-Haq party is part of an alliance of parliamentary opposition groups and represents Yemen's Zaidi Shiites, based in the north of the mainly Sunni Muslim country.
Armed Zaidi rebels have been engaged in sporadic fighting with government forces in northern Yemen since 2004. A ceasefire between the rebels and government forces went into effect on February 12, 2010.