Harf Sofyan , Yemen ( CNN ) -- Security forces opened fire on anti-government protesters in northern Yemen on Friday morning , killing two people and injuring nine others , witnesses said . The forces fired into the protesters in Harf Sufyan city to try and disperse them , witnesses said . Three army planes flying over the crowd also attacked the protesters , witnesses said . The vast majority of the protesters were Houthi rebels , according to Mohammed Abdulsalam , a spokesman for the group . Since 2004 , the Houthis have waged a separatist revolt over who which religious sect will control the area . The Houthis are Shiite Muslims and followers of slain Shiite cleric Hussein al-Houthi . The majority of Yemenis are Sunnis . Elsewhere in Yemen on Friday , thousands of anti-government demonstrators in Sana ' a called for President Ali Abdullah Saleh to step down as they continued to hold a sit-in demonstration outside Sanaa University . The crowd included students , women , tribesmen and members of the opposition In Aden Province . Thousands also continued to demonstrate against Saleh 's regime in the provinces of Taiz , Mareb and Jawf . Hakim Almasmari contributed to this report .
Three army tanks also attacked the protesters , witnesses said . The majority of the protesters are Houthi fighters . The Houthis are Shiite Muslims and followers of slain cleric Hussein al - Houthi .