Thursday's clashes came as Yemenis marked the first anniversary of the election of President Abed Rabbo Mansour Hadi. The election — in a one-candidate referendum vote — was part of U.S.-backed power transfer deal following a year-long uprising that ousted longtime autocratic ruler Ali Abdullah Saleh.
The clashes erupted in the southern city of Aden as pro-Hadi demonstrators gathered to celebrate the anniversary.
Officials say separatist fighters then tried to storm the gathering, prompting security forces to open fire. The officials spoke on condition of anonymity because they were not authorized to speak to the press.
Yemen's north and south were united in 1990.