Yemeni police have been accused of killing a prominent southern separatist in what human rights group Amnesty International called an "execution".
Khaled al-Junaidi, 42, a prominent figure in the Southern Movement, was shot in the chest on Monday when masked security officers in Aden opened fire on his car, AFP news agency said.
He was inspecting protests in the south, organised to press for southern secession, a spokesman for his movement said.
The police had no immediate comment on the death of Junaidi.
A picture of Junaidi on the Southern Movement's Facebook page showed what appeared to be a large bullet hole on the left side of his chest.
This shocking, deliberate killing appears to be an extrajudicial execution
"This shocking, deliberate killing appears to be an extrajudicial execution," Said Boumedouha,deputy director of Amnesty International’s Middle East and North Africa Programme, said.
"Yemeni authorities have an obligation under international law to ensure that an independent, impartial and prompt investigation into this killing is conducted, and that all those responsible are brought to justice, including anyone who ordered the killing."
Radfan al-Dubeis, a spokesman for the Southern Movement, said Junaidi was neither armed nor resisting police, calling the killing "premeditated".
Junaidi, an activist described as a main organiser of the protests against the northern government, was released from prison earlier this month after serving five months for separatist activities and had been preparing to take part in a day of action on Monday.
The south has staged regular sit-ins in central Aden since mid-October demanding secession and accusing the north of negligence and the centralisation of power.
Once-independent, South Yemen unified with the north in 1990. An attempt to break away from the union in 1994 was ruthlessly crushed by former President Ali Abdullah Saleh.