Riot police opened fire into a crowd of retired army officers, killing four people, activists and local medical officials said.
Hundreds of protesters had gathered Saturday in the southern city of Radfan, about 185 miles south of the capital Sana'a, to prepare for a rally scheduled for Sunday to mark the 44th anniversary of southern Yemen's uprising against British occupation.
The protesters were largely former soldiers who fought on the side of the breakaway south in a 1994 civil war.
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